#include<iostream>
#include<queue>
#include<vector>
#include<algorithm>
#include<string>
#include<cstring>
using namespace std;
const int N = 1510;
const int M = 100000;
int D[N];
struct E {
	int to, w, next;
}Edge[M << 1];
int idx, head[N], in[N];
void add(int u, int v, int w) {
	Edge[++idx] = { v,w,head[u]};
	head[u] = idx;
}
void solve()
{
	memset(head, 0, sizeof(head));
	int n, m;
	cin>>n>>m;
	for (int i = 1; i <= m; i++)
	{
		int u, v, w;
                cin>>u>>v>>w;
		add(u, v, w);
		in[v]++;
	}
	queue<int>q;
	q.push(1);
	while (!q.empty())
	{
		int p = q.front();
		q.pop();
		for (int i = head[p]; i; i = Edge[i].next)
		{
			if (--in[Edge[i].to] == 0) q.push(in[Edge[i].to]);
			D[Edge[i].to] = max(D[Edge[i].to], D[Edge[p].to] + Edge[i].w);
		}
	}
	if (D[n]) cout << D[n] << endl;
	else cout << -1 << endl;
}
int main()
{
	solve();
}