#include<iostream>
#include<vector>
#include<queue>
using namespace std;
const int N = 2e3 + 5;

vector<pair<int, int> >edge[N];
queue<int>q;

int dp[N], in[N];

int main(){
	int n, m;
	cin >> n >> m;
	for(int i = 0; i < m; i++){
		int u, v, w;
		cin >> u >> v >> w;
		edge[u].push_back({v, w});
		in[v]++;
	}
	for(int i = 2; i <= n; i++){
		if(in[i] == 0){
			q.push(i);
		}
	}
	while(!q.empty()){
		int h = q.front();
		q.pop();
		for(int i = 0; i < edge[h].size(); i++){
			if(!--(in[edge[h][i].first]))q.push(edge[h][i].first);
		}
	}
	q.push(1);
	while(!q.empty()){
		int h = q.front();
		q.pop();
		for(int i = 0; i < edge[h].size(); i++){
			int v = edge[h][i].first;
			int w = edge[h][i].second;
			dp[v] = max(dp[v], dp[h] + w);
			in[v]--;
			if(in[v] == 0){
				q.push(v);
			} 
		}
	}
	if(dp[n])cout << dp[n];
	else cout << -1;
	return 0;
}