// Sea, You & Me
#include<bits/stdc++.h>
#define LL long long
#define DB double
#define MOD 1000000007
#define ls(x) (x << 1)
#define rs(x) (x << 1 | 1)
#define lowbit(x) ((-x) & x)
#define MP make_pair
#define MT make_tuple
#define VI vector<int>
#define VL vector<LL>
#define VII VI::iterator
#define VLI VL::iterator
#define all(x) x.begin(), x.end()
#define EB emplace_back
#define PII pair<int, int>
#define SI set<int>
#define SII SI::iterator
#define fi first
#define se second
using namespace std;
template<typename T> void chkmn(T &a, const T b) { (a > b) && (a = b); }
template<typename T> void chkmx(T &a, const T b) { (a < b) && (a = b); }
void Inc(int &a, const int &b) { ((a += b) >= MOD) && (a -= MOD); }
void Dec(int &a, const int &b) { ((a -= b) < 0) && (a += MOD); }
void Mul(int &a, const int &b) { a = 1LL * a * b % MOD; }
void Sqr(int &a) { a = 1LL * a * a % MOD; }
int inc(const int &a, const int &b) { return (a + b >= MOD) ? a + b - MOD : a + b; }
int dec(const int &a, const int &b) { return (a - b < 0) ? a - b + MOD : a - b; }
int mul(const int &a, const int &b) { return 1LL * a * b % MOD; }
int sqr(const int &a) { return 1LL * a * a % MOD; }
int qwqmi(int x, int k = MOD - 2)
{
	int res = 1;
	while(k)
	{
		if(k & 1) Mul(res, x);
		k >>= 1, Sqr(x);
	}
	return res;
}
template<typename T> void read(T &x)
{
	x = 0;
	int f = 1;
	char ch = getchar();
	while(!isdigit(ch))
	{
		if(ch == '-')
			f = -1;
		ch = getchar();
	}
	while(isdigit(ch))
	{
		x = (x << 1) + (x << 3) + (ch ^ 48);
		ch = getchar();
	}
	x = x * f;
}
const int N = 3e5 + 5;
const int F = 21;
int n, m;
LL ans = 0, tot = 0;
vector<PII> G[N]; 
vector<PII> T[N];
struct Saratoga
{
	int u, v, w;	
}ed[N];
struct Essex
{
	int fath[N], sz[N];
	void insert(int x)
	{
		fath[x] = x;
		sz[x] = 1;
	}
	void init()
	{
		for(int i = 1; i <= n; ++i)
			insert(i);
	}
	int find(int x)
	{
		if(fath[x] != x)
			fath[x] = find(fath[x]);
		return fath[x];
	}
	bool same(int x, int y)
	{
		return find(x) == find(y);
	}
	void unite(int x, int y, int w)
	{
		if(same(x, y))
			return;
		T[x].EB(MP(y, w));
		T[y].EB(MP(x, w));
		x = find(x);
		y = find(y);
		ans += 1LL * w * sz[x] * sz[y];
		tot += 1LL * sz[x] * sz[y];
		fath[y] = x;
		sz[x] += sz[y];
	}
}esx;
int minw[N][F], f[N][F], depth[N];
void dfs(int u, int fa)
{
	f[u][0] = fa;
	depth[u] = depth[fa] + 1;
	for(int i = 1; i < F; ++i)
	{
		f[u][i] = f[f[u][i - 1]][i - 1];
		minw[u][i] = min(minw[u][i - 1], minw[f[u][i - 1]][i - 1]);
	}
	for(auto nxt : T[u])
	{
		int v = nxt.fi;
		int w = nxt.se;
		if(v == fa) continue;
		minw[v][0] = w;
		dfs(v, u);
	}
}
int get(int x, int y)
{
	if(depth[x] < depth[y])
		swap(x, y);
	int res = 1e7 + 5;
	for(int i = F - 1; i >= 0; --i)
		if(depth[f[x][i]] >= depth[y])
		{
			chkmn(res, minw[x][i]);
			x = f[x][i];
		}
	if(x == y) return res;
	for(int i = F - 1; i >= 0; --i)
		if(f[x][i] ^ f[y][i])
		{
			chkmn(res, minw[x][i]);
			chkmn(res, minw[y][i]);
			x = f[x][i];
			y = f[y][i];
		}
	chkmn(res, minw[x][0]);
	chkmn(res, minw[y][0]);
	return res;
}
int main()
{
	read(n), read(m);
	esx.init();
	for(int i = 1; i <= m; ++i)
	{
		int x, y, z;
		read(x), read(y), read(z);
		ed[i] = (Saratoga){x, y, z};
		G[x].EB(MP(y, z));
		G[y].EB(MP(x, z));
	}
	sort(ed + 1, ed + m + 1, [&](Saratoga A, Saratoga B){
		return A.w > B.w;
	});
	for(int i = 1; i <= m; ++i)
	{
		int u = ed[i].u, v = ed[i].v, w = ed[i].w;
		if(esx.same(u, v)) continue;
		esx.unite(u, v, w); 
	}
	ans += 1LL * n * (n - 1) / 2 - tot;
	
	for(int i = 0; i <= n; ++i)
		for(int j = 0; j < F; ++j)
			minw[i][j] = 1e7 + 5;
	for(int i = 1; i <= n; ++i)
		if(!depth[i]) dfs(i, 0);
	for(int i = 1; i <= m; ++i)
	{
		int u = ed[i].u, v = ed[i].v;
		int res = get(u, v);
		if(res != ed[i].w)
			return puts("-1"), 0;
	}
	printf("%lld\n", ans);
	return 0;
}

/* sample 1
4 2
1 2 5
2 4 3
ans : 14
*/

/* sample 2
4 4
1 2 10
1 3 20
2 4 30
3 4 40
ans : -1
*/