#include<iostream>
#include<vector>
#include<algorithm>
#include<queue>

using namespace std;
const int N = 2e5 + 7;
#define int long long

vector<pair<int, int>> G[N];
int in[N] = {0};
int vis[N] = {0};
int w[N] = {0};

void solve() {
    int n, m;
    cin >> n >> m;
    for (int i = 1; i <= m; i++) {
        int x, y, z;
        cin >> x >> y >> z;
        G[y].push_back({x, z});
        G[z].push_back({x, y});
        in[x] += 2;
    }
    queue<int> Q;
    for (int i = 1; i <= n; i++) {
        if (in[i] == 0) {
            Q.push(i);
            w[i] = 1;
        }
    }
    while (!Q.empty()) {
        int u = Q.front();
        vis[u] = 1;
        Q.pop();
        for (auto [to, fri]: G[u]) {
            if (w[fri]) {
                w[to] = max(w[to], w[u] + w[fri]);
                in[to]--;
                if (in[to] == 0)Q.push(to);
            }
        }
    }
    int sum = 0;
    for (int i = 1; i <= n; i++) {
        sum += w[i];
        if (w[i] == 0) {
            cout << -1 << endl;
            return;
        }
    }
    cout << sum << endl;

}

signed main() {
  //  freopen("D:\\Development_Software\\CLion\\CLionProjects\\Test\\in", "r", stdin);
    ios::sync_with_stdio(false);
    int T = 1;
    while (T--)solve();
}