我用floyd负数跑的，#7 WA了，求大佬帮忙看看哪里有问题。

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <vector>
#define int long long
#define FAST ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
using namespace std;
int e[1503][1503]; 

signed main(){
	FAST; 

	int n, m;
    cin >> n >> m;
    for(int i = 1; i <= n; ++i){
		for(int j = 1; j <= n; ++j){
			if(i == j){
				e[i][j] = 0;
			}else{
				e[i][j] = 1e9;
			}
		}
	}
    for(int i = 1; i <= m; ++i){
    	int u, v, w;
    	cin >> u >> v >> w;
    	e[u][v] = min(e[u][v], -w);
	}
	for(int k = 1; k <= n; ++k){
		for(int i = 1; i <= n; ++i){
			for(int j = 1; j <= n; ++j){
				if(e[i][j] > e[i][k] + e[k][j]){
					e[i][j] = e[i][k] + e[k][j];
				}
			}
		}
	}
	if(e[1][n] != 1e9){
		cout << -e[1][n];
	}else{
		cout << -1;
	}
	return 0;
}