#include<bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(int i=(a);i<=(b);i++)
#define per(i,a,b) for(int i=(a);i>=(b);i--)
#define rept(i,a,ne) for(int i=(a);i!=-1;i=ne[i])

#define debug(x) cout<<#x<<" :"<<x<<endl;
// #define endl '\n'
typedef long long LL;
const int N=4e5+10;
int h[N],ne[N],e[N],idx;
LL w[N],sz[N];
void add(int x,int y)
{
    e[idx]=y,ne[idx]=h[x],h[x]=idx++;
}
int d[N];
int ans[N];

void slove()
{
    memset(h,-1,sizeof h);
    int n,m;
    cin>>n>>m;
    rep(i,1,m)
    {
        int x,y,z;
        cin>>x>>y>>z;
        add(y,x);
        add(z,x);
        d[x]+=2;
    }
    int num=0;
    queue<int>qu;
    rep(i,1,n)
    {
        if(d[i]==0)
            qu.push(i),num++,ans[i]=1;
    }
    LL sum=0;
    while (qu.size())
    {
        int x=qu.front();
        sum+=ans[x];
        qu.pop();
        rept(i,h[x],ne)
        {
            int y=e[i];
            ans[y]+=ans[x];
            if(--d[y]==0)
            {
                qu.push(y),num++;
            }
        }
    }
    // debug(num);
    if(num==n)
        cout<<sum<<endl;
    else
        cout<<-1<<endl;
}
int main()
{
#ifndef ONLINE_JUDGE
    freopen("test.in", "r", stdin);
    freopen("test.out", "w", stdout);
#endif
    int t=1;
     // cin>>t;
    while(t--)
    {
        slove();
    }
    return 0;
}