#include <bits/stdc++.h>
#define int long long 
using namespace std;
const int N=200010,mod=1e9;
vector<pair<int,int> > h[N];
int dian[N];
int idx;
int q[N];
int f[N];
int n,m;
void tp(){
    for(int i=1;i<=n;i++){
        if(dian[i]==0){
            //cout<<i<<" ";
            q[++idx]=i;
            f[i]=1;
        }
    }
    //cout<<idx;
    for(int i=1;i<=idx;i++){
        int u=q[i];
        int w=f[u];
        for(int j=0;j<h[u].size();j++){
            int fu=h[u][j].first,c=h[u][j].second;
            if(f[c]!=0){
                f[fu]=min(mod,max(f[fu],w+f[c]));
                dian[fu]--;
                if(dian[fu]==0) q[++idx]=fu;
            } 
        }
    }
}


signed main(){

    cin>>n>>m;
    while(m--){
        int a,b,c;
        cin>>a>>b>>c;
        h[b].push_back({a,c});
        h[c].push_back({a,b});
        dian[a]+=2;
    }

    tp();
   // cout<<idx<<" ";
    if(idx!=n){
        cout<<-1;
        return 0;
    }
    int ans=0;
    for(int i=1;i<=n;i++) ans+=f[i];
    if(ans>1e9) cout<<-1;
    else cout<<ans;
}