#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
void solve() {
    int n,m;
    cin >> n >> m;
    vector g(n,vector<int>(0));
    vector<int>num(n);
    for ( int i = 0 ; i < m ; i++ ) {
        int x,y,z;
        cin >> x >> y >> z;
        x--,y--,z--;
        g[x].emplace_back(y);
        g[x].emplace_back(z);
        num[y]++;
        num[z]++;
    }
    i64 ans = 0;
    vector<i64>vis(n);
    int f = 0;
    auto dfs = [&](auto& dfs, int p)->void{
        i64 res = 0;
        for ( auto q : g[p] ) {
            if(!vis[q]) {
                dfs(dfs,q);
            }
            res += vis[q];
        }
        if ( vis[p] > 1000000000 ) f = 1;
        vis[p] = max(res,1LL);
    };
    
    for ( int i = 0 ; i < n ; i++ ) {
        if ( num[i] == 0 ) dfs(dfs,i);
    }
    for ( int i = 0 ; i < n ; i++ ) {
        if ( !vis[i] ) f = 1;
        ans += vis[i];
    }
    if ( f ) {
        cout << -1 << '\n';
        return;
    }
    if ( ans > 1000000000 ) cout << -1 << '\n';
    else cout << ans << '\n';
}
int main() {
    ios::sync_with_stdio(0);
    cin.tie(0), cout.tie(0);
    int T = 1;
    // cin >> T;
    while ( T-- ) {
        solve(); 
    }
}