#include<bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef double db;
#define int long long
const int N = 2e5 + 5;
#define endl '\n'
vector<vector<int>> q1, q2;
int vis[N];
int f = 0;

void dfs(int u) {
    vis[u] = 1;
    for (int i = 0; i < q2[u].size(); i++) {
        int y = q2[u][i];
        if (vis[y]) {
            f = 1;
            return;
        }
        dfs(y);
    }
}

int in[N];
int w[N];
void solve() {
    int n, m;
    cin >> n >> m;
    q1 = vector<vector<int>>(n + 5);
    q2 = vector<vector<int>>(n + 5);
    for (int i = 1, a, b, c; i <= m; i++) {
        cin >> a >> b >> c;
        q1[b].push_back(a);
        q1[c].push_back(a);
        q2[a].push_back(b);
        q2[a].push_back(c);
        in[a] += 2;
    }
    for (int i = 1; i <= n; i++) {
        if (vis[i])continue;
        dfs(i);
    }
    if (f) {
        cout << -1 << '\n';
        return;
    }
    queue<int>q;
    int ans=0;
    for (int i = 1; i <= n;i++){
        if(in[i]==0){q.push(i);w[i]=1;}
    }
    while(!q.empty()){
        int y=q.front();
        q.pop();
        for(int i=0;i<q1[y].size();i++){
            int y1=q1[y][i];
            in[y1]--;
            w[y1]+=w[y];
            if(!in[y1]){
                q.push(y1);
            }
        }
    }
    for(int i=1;i<=n;i++){
      //  cout<<w[i]<<" ";
        ans+=w[i];
    }
     if(ans<=1e9){
        cout<<ans<<'\n';
    }else{
        cout<<-1<<'\n';
    }

}

signed main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    //cout << fixed << setprecision(5);
    int T = 1;
    //cin >> T;
    while (T--) solve();
    return 0;
}