#include <iostream>
#include <vector>
using namespace std;

const int NEG_INF = -1e9;

int main() {
    int v, e;
    cin >> v >> e;

    vector<vector<int>> dp(v, vector<int>(v, NEG_INF));
    for (int i = 0; i < v; i++) dp[i][i] = 0;  // 自环=0

    for (int i = 0; i < e; i++) {
        int a, b, w;
        cin >> a >> b >> w;
        dp[a - 1][b - 1] = max(dp[a - 1][b - 1], w); // 可能多条边，取最大
    }

    for (int k = 0; k < v; k++) {
        for (int i = 0; i < v; i++) {
            if (dp[i][k] <= NEG_INF / 2) continue;
            for (int j = 0; j < v; j++) {
                if (dp[k][j] <= NEG_INF / 2) continue;
                dp[i][j] = max(dp[i][j], dp[i][k] + dp[k][j]);
            }
        }
    }

    int mx = -1;
    for (int i = 0; i < v; i++) {
        for (int j = 0; j < v; j++) {
            if (dp[i][j] > NEG_INF / 2) {
                mx = max(mx, dp[i][j]);
            }
        }
    }

    mx == -1 ? cout << mx << endl;
    return 0;
}