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

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);
    map<pair<int, int>, int> mp1, mp2;
    for (int i = 1, a, b, c; i <= m; i++) {
        cin >> a >> b >> c;
        if (mp1[{b, a}] == 0) {
            q1[b].push_back(a);
            mp1[{b, a}] = c;
        }
        if (mp1[{c, a}] == 0) {
            q1[c].push_back(a);
            mp1[{c, a}] = b;
        }
        if (mp2[{a, b}] == 0) {
            q2[a].push_back(b);
            mp2[{a, b}] = 1;
            in[a]++;
        }
        if (mp2[{a, c}] == 0) {
            q2[a].push_back(c);
            mp2[{a, c}] = 1;
            in[a]++;
        }
    }
    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];
            // cout<<y1<<" "<<y<<"\n";
            in[y1]--;
            w[y1] = max(w[y1], (w[y] + w[mp1[{y, y1}]]));
            if (!in[y1]) {
                q.push(y1);
            }
        }
    }
    for (int i = 1; i <= n; i++) {
        //   cout<<w[i]<<" ";
        if(w[i]==0)f=1;
        ans += w[i];
    }
    if(f){
        cout << -1 << '\n';
        return;
    }
    if (ans <= 1e9) {
        cout << ans << '\n';
    } else {
        cout << -1 << '\n';
    }
//    cout<<ans<<'\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;
}