//#pragma GCC optimize ("Ofast")
//#pragma GCC optimize ("unroll-loops")
#include <bits/stdc++.h>
using namespace std;

#define fi first
#define se second
typedef pair<int, int> pii;
typedef long long ll;
typedef long double ld;
//mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());

const int mod = 998244353;
const int N = 2e5 + 5;

int n, m;
vector<int> G[N];
int in[N];
ll dp[N];
map<int, int> mp[N];
void MAIN() {
    cin >> n >> m;
    for (int i = 1; i <= m; i++) {
        int a, b, c;
        cin >> a >> b >> c;
        G[a].push_back(c);
        G[b].push_back(c);
        in[c]++;
    }
    queue<int> q;
    for (int i = 1; i <= n; i++) {
        if (!in[i]) q.push(i), dp[i] = 1;
    }
    while (!q.empty()) {
        int u = q.front(); q.pop();
        for (int v : G[u]) {
            if (!mp[v].count(u)) {
                mp[v][u] = 1;
                dp[v] += dp[u];
            }
            --in[v];
            if (!in[v]) q.push(v);
        }
    }
    ll sum = 0;
    for (int i = 1; i <= n; i++) {
        if (dp[i] == 0) return cout << "-1\n", void();
        sum += dp[i];
    }
    if (sum > 1000000000ll) cout << -1 << '\n';
    else cout << sum << '\n';
}

int main() {
#ifdef LOCAL
    auto start = chrono::steady_clock::now();
    freopen("miku.in", "r", stdin);
    freopen("miku.out", "w", stdout);
#endif
    ios::sync_with_stdio(0);
    cin.tie(0);
    int T = 1;
    // cin >> T;
    while (T--) MAIN();
#ifdef LOCAL
    auto end = chrono::steady_clock::now();
    cout << "\nqwq: " <<  chrono::duration_cast<chrono::milliseconds>(end - start).count() << "ms\n";
#endif
    return 0;
}