from collections import defaultdict

class Graph:
    def __init__(self,V,edges=None,graph=None,directed=False,weighted=False,inf=float("inf")):
        self.V=V
        self.directed=directed
        self.weighted=weighted
        self.inf=inf
        if graph!=None:
            self.graph=graph
            """
            self.edges=[]
            for i in range(self.V):
                if self.weighted:
                    for j,d in self.graph[i]:
                        if self.directed or not self.directed and i<=j:
                            self.edges.append((i,j,d))
                else:
                    for j in self.graph[i]:
                        if self.directed or not self.directed and i<=j:
                            self.edges.append((i,j))
            """
        else:
            self.edges=edges
            self.graph=[[] for i in range(self.V)]
            if weighted:
                for i,j,d in self.edges:
                    self.graph[i].append((j,d))
                    if not self.directed:
                        self.graph[j].append((i,d))
            else:
                for i,j in self.edges:
                    self.graph[i].append(j)
                    if not self.directed:
                        self.graph[j].append(i)

    def Kruskal(self,maximize=False,spanning_tree=False):
        UF=UnionFind(self.V)
        sorted_edges=sorted(self.edges if self.weighted else [(x,y,1) for x,y in self.edges],key=lambda tpl:tpl[2],reverse=maximize)
        if spanning_tree:
            st=[]
        else:
            cost=0
        for x,y,d in sorted_edges:
            if not UF.Same(x,y):
                UF.Union(x,y)
                if spanning_tree:
                    st.append((x,y,d))
                else:
                    cost+=d
        return st if spanning_tree else cost

    def SIV_DFS(self,s,bipartite_graph=False,cycle_detection=False,directed_acyclic=False,euler_tour=False,linked_components=False,lowlink=False,parents=False,postorder=False,preorder=False,subtree_size=False,topological_sort=False,unweighted_dist=False,weighted_dist=False):
        seen=[False]*self.V
        finished=[False]*self.V
        if directed_acyclic or cycle_detection or topological_sort:
            dag=True
        if euler_tour:
            et=[]
        if linked_components:
            lc=[]
        if lowlink:
            order=[None]*self.V
            ll=[None]*self.V
            idx=0
        if parents or cycle_detection or lowlink or subtree_size:
            ps=[None]*self.V
        if postorder or topological_sort:
            post=[]
        if preorder:
            pre=[]
        if subtree_size:
            ss=[1]*self.V
        if unweighted_dist or bipartite_graph:
            uwd=[self.inf]*self.V
            uwd[s]=0
        if weighted_dist:
            wd=[self.inf]*self.V
            wd[s]=0
        stack=[(s,0)] if self.weighted else [s]
        while stack:
            if self.weighted:
                x,d=stack.pop()
            else:
                x=stack.pop()
            if not seen[x]:
                seen[x]=True
                stack.append((x,d) if self.weighted else x)
                if euler_tour:
                    et.append(x)
                if linked_components:
                    lc.append(x)
                if lowlink:
                    order[x]=idx
                    ll[x]=idx
                    idx+=1
                if preorder:
                    pre.append(x)
                for y in self.graph[x]:
                    if self.weighted:
                        y,d=y
                    if not seen[y]:
                        stack.append((y,d) if self.weighted else y)
                        if parents or cycle_detection or lowlink or subtree_size:
                            ps[y]=x
                        if unweighted_dist or bipartite_graph:
                            uwd[y]=uwd[x]+1
                        if weighted_dist:
                            wd[y]=wd[x]+d
                    elif not finished[y]:
                        if (directed_acyclic or cycle_detection or topological_sort) and dag:
                            dag=False
                            if cycle_detection:
                                cd=(y,x)
            elif not finished[x]:
                finished[x]=True
                if euler_tour:
                    et.append(~x)
                if lowlink:
                    bl=True
                    for y in self.graph[x]:
                        if self.weighted:
                            y,d=y
                        if ps[x]==y and bl:
                            bl=False
                            continue
                        ll[x]=min(ll[x],order[y])
                    if x!=s:
                        ll[ps[x]]=min(ll[ps[x]],ll[x])
                if postorder or topological_sort:
                    post.append(x)
                if subtree_size:
                    for y in self.graph[x]:
                        if self.weighted:
                            y,d=y
                        if y==ps[x]:
                            continue
                        ss[x]+=ss[y]
        if bipartite_graph:
            bg=[[],[]]
            for tpl in self.edges:
                x,y=tpl[:2] if self.weighted else tpl
                if uwd[x]==self.inf or uwd[y]==self.inf:
                    continue
                if not uwd[x]%2^uwd[y]%2:
                    bg=False
                    break
            else:
                for x in range(self.V):
                    if uwd[x]==self.inf:
                        continue
                    bg[uwd[x]%2].append(x)
        retu=()
        if bipartite_graph:
            retu+=(bg,)
        if cycle_detection:
            if dag:
                cd=[]
            else:
                y,x=cd
                cd=self.Route_Restoration(y,x,ps)
            retu+=(cd,)
        if directed_acyclic:
            retu+=(dag,)
        if euler_tour:
            retu+=(et,)
        if linked_components:
            retu+=(lc,)
        if lowlink:
            retu=(ll,)
        if parents:
            retu+=(ps,)
        if postorder:
            retu+=(post,)
        if preorder:
            retu+=(pre,)
        if subtree_size:
            retu+=(ss,)
        if topological_sort:
            if dag:
                tp_sort=post[::-1]
            else:
                tp_sort=[]
            retu+=(tp_sort,)
        if unweighted_dist:
            retu+=(uwd,)
        if weighted_dist:
            retu+=(wd,)
        if len(retu)==1:
            retu=retu[0]
        return retu

    def Build_LCA(self,s,segment_tree=False):
        self.lca_segment_tree=segment_tree
        if self.lca_segment_tree:
            self.lca_euler_tour,self.lca_parents,depth=self.SIV_DFS(s,euler_tour=True,parents=True,unweighted_dist=True)
            self.lca_dfs_in_index=[None]*self.V
            self.lca_dfs_out_index=[None]*self.V
            for i,x in enumerate(self.lca_euler_tour):
                if x>=0:
                    self.lca_dfs_in_index[x]=i
                else:
                    self.lca_dfs_out_index[~x]=i
            self.ST=Segment_Tree(2*self.V,lambda x,y:min(x,y),self.V)
            lst=[None]*(2*self.V)
            for i in range(2*self.V-1):
                if self.lca_euler_tour[i]>=0:
                    lst[i]=depth[self.lca_euler_tour[i]]
                else:
                    lst[i]=depth[self.lca_parents[~self.lca_euler_tour[i]]]
            lst[2*self.V-1]=-1
            self.ST.Build(lst)
        else:
            self.lca_parents,self.lca_depth=self.SIV_DFS(s,parents=True,unweighted_dist=True)
            self.lca_PD=Path_Doubling(self.V,self.lca_parents)
            self.lca_PD.Build_Next(self.V)

    def LCA(self,a,b):
        if self.lca_segment_tree:
            m=min(self.lca_dfs_in_index[a],self.lca_dfs_in_index[b])
            M=max(self.lca_dfs_in_index[a],self.lca_dfs_in_index[b])
            x=self.lca_euler_tour[self.ST.Fold_Index(m,M+1)]
            if x>=0:
                lca=x
            else:
                lca=self.lca_parents[~x]
        else:
            if self.lca_depth[a]>self.lca_depth[b]:
                a,b=b,a
            b=self.lca_PD.Permutation_Doubling(b,self.lca_depth[b]-self.lca_depth[a])
            if a!=b:
                for k in range(self.lca_PD.k-1,-1,-1):
                    if self.lca_PD.permutation_doubling[k][a]!=self.lca_PD.permutation_doubling[k][b]:
                        a,b=self.lca_PD.permutation_doubling[k][a],self.lca_PD.permutation_doubling[k][b]
                a,b=self.lca_PD.permutation_doubling[0][a],self.lca_PD.permutation_doubling[0][b]
            lca=a
        return lca

class Segment_Tree:
    def __init__(self,N,f,e,lst=None,dynamic=False):
        self.f=f
        self.e=e
        self.N=N
        if dynamic:
            self.segment_tree=defaultdict(lambda:self.e)
        else:
            if lst==None:
                self.segment_tree=[self.e]*2*self.N
            else:
                assert len(lst)<=self.N
                self.segment_tree=[self.e]*self.N+[x for x in lst]+[self.e]*(N-len(lst))
                for i in range(self.N-1,0,-1):
                    self.segment_tree[i]=self.f(self.segment_tree[i<<1],self.segment_tree[i<<1|1])

    def __getitem__(self,i):
        if type(i)==int:
            if -self.N<=i<0:
                return self.segment_tree[i+self.N*2]
            elif 0<=i<self.N:
                return self.segment_tree[i+self.N]
            else:
                raise IndexError("list index out of range")
        else:
            a,b,c=i.start,i.stop,i.step
            if a==None:
                a=self.N
            else:
                a+=self.N
            if b==None:
                b=self.N*2
            else:
                b+=self.N
            return self.segment_tree[slice(a,b,c)]

    def __setitem__(self,i,x):
        if -self.N<=i<0:
            i+=self.N*2
        elif 0<=i<self.N:
            i+=self.N
        else:
            raise IndexError("list index out of range")
        self.segment_tree[i]=x
        while i>1:
            i>>= 1
            self.segment_tree[i]=self.f(self.segment_tree[i<<1],self.segment_tree[i<<1|1])

    def Build(self,lst):
        for i,x in enumerate(lst,self.N):
            self.segment_tree[i]=x
        for i in range(self.N-1,0,-1):
            self.segment_tree[i]=self.f(self.segment_tree[i<<1],self.segment_tree[i<<1|1])

    def Fold(self,L=None,R=None):
        if L==None:
            L=self.N
        else:
            L+=self.N
        if R==None:
            R=self.N*2
        else:
            R+=self.N
        vL=self.e
        vR=self.e
        while L<R:
            if L&1:
                vL=self.f(vL,self.segment_tree[L])
                L+=1
            if R&1:
                R-=1
                vR=self.f(self.segment_tree[R],vR)
            L>>=1
            R>>=1
        return self.f(vL,vR)

    def Fold_Index(self,L=None,R=None):
        if L==None:
            L=self.N
        else:
            L+=self.N
        if R==None:
            R=self.N*2
        else:
            R+=self.N
        if L==R:
            return None
        x=self.Fold(L-self.N,R-self.N)
        while L<R:
            if L&1:
                if self.segment_tree[L]==x:
                    i=L
                    break
                L+=1
            if R&1:
                R-=1
                if self.segment_tree[R]==x:
                    i=R
                    break
            L>>=1
            R>>=1
        while i<self.N:
            if self.segment_tree[i]==self.segment_tree[i<<1]:
                i<<=1
            else:
                i<<=1
                i|=1
        i-=self.N
        return i

    def Bisect_Right(self,L=None,f=None):
        if L==self.N:
            return self.N
        if L==None:
            L=0
        L+=self.N
        vl=self.e
        vr=self.e
        l,r=L,self.N*2
        while l<r:
            if l&1:
                vl=self.f(vl,self.segment_tree[l])
                l+=1
            if r&1:
                r-=1
                vr=self.f(self.segment_tree[r],vr)
            l>>=1
            r>>=1
        if f(self.f(vl,vr)):
            return self.N
        v=self.e
        while True:
            while L%2==0:
                L>>=1
            vv=self.f(v,self.segment_tree[L])
            if f(vv):
                v=vv
                L+=1
            else:
                while L<self.N:
                    L<<=1
                    vv=self.f(v,self.segment_tree[L])
                    if f(vv):
                        v=vv
                        L+=1
                return L-self.N

    def Bisect_Left(self,R=None,f=None):
        if R==0:
            return 0
        if R==None:
            R=self.N
        R+=self.N
        vl=self.e
        vr=self.e
        l,r=self.N,R
        while l<r:
            if l&1:
                vl=self.f(vl,self.segment_tree[l])
                l+=1
            if r&1:
                r-=1
                vr=self.f(self.segment_tree[r],vr)
            l>>=1
            r>>=1
        if f(self.f(vl,vr)):
            return 0
        v=self.e
        while True:
            R-=1
            while R>1 and R%2:
                R>>=1
            vv=self.f(self.segment_tree[R],v)
            if f(vv):
                v=vv
            else:
                while R<self.N:
                    R=2*R+1
                    vv=self.f(self.segment_tree[R],v)
                    if f(vv):
                        v=vv
                        R-=1
                return R+1-self.N

    def __str__(self):
        return "["+", ".join(map(str,self.segment_tree[self.N:]))+"]"

class UnionFind:
    def __init__(self,N,label=None,f=None,weighted=False,rollback=False):
        self.N=N
        self.parents=[None]*self.N
        self.size=[1]*self.N
        self.roots={i for i in range(self.N)}
        self.label=label
        if self.label!=None:
            self.label=[x for x in label]
        self.f=f
        self.weighted=weighted
        if self.weighted:
            self.weight=[0]*self.N
        self.rollback=rollback
        if self.rollback:
            self.operate_list=[]
            self.operate_set=[]

    def Find(self,x):
        stack=[]
        while self.parents[x]!=None:
            stack.append(x)
            x=self.parents[x]
        if not self.rollback:
            if self.weighted:
                w=0
                for y in stack[::-1]:
                    self.parents[y]=x
                    w+=self.weight[y]
                    self.weight[y]=w
            else:
                for y in stack[::-1]:
                    self.parents[y]=x
        return x

    def Union(self,x,y,w=None):
        root_x=self.Find(x)
        root_y=self.Find(y)
        if self.rollback:
            self.operate_list.append([])
            self.operate_set.append([])
        if root_x==root_y:
            if self.weighted:
                if self.weight[y]-self.weight[x]==w:
                    return True
                else:
                    return False
        else:
            if self.size[root_x]<self.size[root_y]:
                x,y=y,x
                root_x,root_y=root_y,root_x
                if self.weighted:
                    w=-w
            if self.rollback:
                self.operate_list[-1].append((self.parents,root_y,self.parents[root_y]))
                self.operate_list[-1].append((self.size,root_x,self.size[root_x]))
                self.operate_set[-1].append(root_y)
                if self.label!=None:
                    self.operate_list[-1]((self.label,root_x,self.label[root_x]))
                if self.weighted:
                    self.operate_list[-1].append((self.weight,root_y,self.weight[root_y]))
            self.parents[root_y]=root_x
            self.size[root_x]+=self.size[root_y]
            self.roots.remove(root_y)
            if self.label!=None:
                self.label[root_x]=self.f(self.label[root_x],self.label[root_y])
            if self.weighted:
                self.weight[root_y]=w+self.weight[x]-self.weight[y]

    def Size(self,x):
        return self.size[self.Find(x)]

    def Same(self,x,y):
        return self.Find(x)==self.Find(y)

    def Label(self,x):
        return self.label[self.Find(x)]

    def Weight(self,x,y):
        root_x=self.Find(x)
        root_y=self.Find(y)
        if root_x!=root_y:
            return None
        return self.weight[y]-self.weight[x]

    def Roots(self):
        return list(self.roots)

    def Linked_Components_Count(self):
        return len(self.roots)

    def Linked_Components(self):
        linked_components=defaultdict(list)
        for x in range(self.N):
            linked_components[self.Find(x)].append(x)
        return linked_components

    def Rollback(self):
        assert self.rollback
        if self.operate_list:
            for lst,x,v in self.operate_list.pop():
                lst[x]=v
            for x in self.operate_set.pop():
                self.roots.add(x)            
            return True
        else:
            return False

    def __str__(self):
        linked_components=defaultdict(list)
        for x in range(self.N):
            linked_components[self.Find(x)].append(x)
        return "\n".join(f"{r}: {linked_components[r]}" for r in sorted(list(linked_components.keys())))

class Path_Doubling:
    def __init__(self,N,permutation,lst=None,f=None,e=None):
        self.N=N
        self.permutation=permutation
        self.lst=lst
        self.f=f
        self.e=e

    def Build_Next(self,K=None):
        if K==None:
            K=self.N
        self.k=K.bit_length()
        self.permutation_doubling=[[None]*self.N for k in range(self.k)]
        for n in range(self.N):
            self.permutation_doubling[0][n]=self.permutation[n]
        if self.lst!=None:
            self.doubling=[[self.e]*self.N for k in range(self.k)]
            for n in range(self.N):
                self.doubling[0][n]=self.lst[n]
        for k in range(1,self.k):
            for n in range(self.N):
                if self.permutation_doubling[k-1][n]!=None:
                    self.permutation_doubling[k][n]=self.permutation_doubling[k-1][self.permutation_doubling[k-1][n]]
                    if self.f!=None:
                        self.doubling[k][n]=self.f(self.doubling[k-1][n],self.doubling[k-1][self.permutation_doubling[k-1][n]])

    def Permutation_Doubling(self,N,K):
        if K<0 or 1<<self.k<=K:
            return None
        for k in range(self.k):
            if K>>k&1 and N!=None:
                N=self.permutation_doubling[k][N]
        return N

    def Doubling(self,N,K):
        if K<0:
            return self.e
        retu=self.e
        for k in range(self.k):
            if K>>k&1:
                if self.permutation_doubling[k][N]==None:
                    return None
                retu=self.f(retu,self.doubling[k][N])
                N=self.permutation_doubling[k][N]
        return N,retu

    def Bisect(self,x,is_ok):
        if not is_ok(x):
            return -1,None
        K=0
        for k in range(self.k-1,-1,-1):
            if is_ok(self.permutation_doubling[k][x]):
                K|=1<<k
                x=self.permutation_doubling[k][x]
        return K,x

N,M=map(int,input().split())
edges=[]
for m in range(M):
    u,v,w=map(int,input().split())
    u-=1;v-=1
    edges.append((u,v,w))

T0=Graph(N,edges=edges,weighted=True).Kruskal(spanning_tree=True)
set_T0=set(T0)
T1=Graph(N,edges=[(u,v,w) for u,v,w in edges if not(u,v,w) in set_T0],weighted=True).Kruskal(spanning_tree=True)
if len(T1)<N-1:
    ans=-1
else:
    T1=Graph(N,edges=T1,weighted=True)
    T1.Build_LCA(0)
    depth=T1.lca_depth
    parents=[0]+T1.lca_parents[1:]
    inf=1<<30
    lst=[-inf]*N
    for u in range(N):
        for v,w in T1.graph[u]:
            if v==parents[u]:
                continue
            lst[v]=w
    PD=Path_Doubling(N,permutation=parents,lst=lst,f=max,e=-inf)
    PD.Build_Next()
    ans=0
    for u,v,w in T0:
        lca=T1.LCA(u,v)
        ma=max(PD.Doubling(u,depth[u]-depth[lca])[1],PD.Doubling(v,depth[v]-depth[lca])[1])
        ans+=ma+1-w

print(ans)