#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 1e4+10,M=2e4+10;
int to[N],h[N],e[M],ne[M],idx;
ll n, m,cnt;
ll dist[N];
ll w[N];
void add(ll a,ll b)
{
    e[idx] = b;
    ne[idx] = h[a];
    h[a] = idx++;
}

bool topsort()
{
     cnt = 0;
    queue<int>q;
    for (int i = 1; i <= n; i++)
    {
        if (!to[i])
        {
            q.push(i);
            cnt++;
            w[i] = 1;
        }     
    }
    while (!q.empty())
    {
        auto u = q.front();
        q.pop();
        for (int i = h[u]; ~i; i=ne[i])
        {
            ll j = e[i];
            w[j] += w[u];
            if (--to[j] == 0)
            {
                q.push(j);
                cnt++;
            }
        }
    }

    return cnt == n ;
}


int main()
{
    ios::sync_with_stdio(0), cin.tie(0);
    memset(h, -1, sizeof(h));
    cin >> n >> m;
    ll a, b, c;
    for (int i = 0; i < m; i++)
    {
        cin >>a>> b>> c;
        add(b, a);
        add(c, a);
        to[a]++;
        to[a]++;
    }
    
    if (!topsort())cout << "-1" << endl;
    else
    {
        ll sum = 0;
        for (int i =1; i <=cnt; i++)
        {
           // cout << w[i] << " ";
            sum += w[i];
        }
       // cout << endl;
        cout << sum << endl;
    }

}