#5 #6 WA了，其他全对，拓扑排序做的，求大佬助调.
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#include <bits/stdc++.h>
using namespace std;

const int N = 10010;

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

int d[N];
int dis[N];

int main()
{
	int n, m;
	scanf("%d%d", &n, &m);
	for (int i = 1; i <= m; i++)
	{
		int u, v, w;
		scanf("%d%d%d", &u, &v, &w);
		g[u].push_back(make_pair(v, w));
		d[v]++;
	}
	for (int i = 1; i <= n; i++)
	{
		if (d[i] == 0)
		{
			q.push(i);
		}
	}
	for (int i = 1; i <= n; i++)
	{
		dis[i] = -1;
	}
	dis[1] = 0;
	while (!q.empty())
	{
		int u = q.front(); q.pop();
		for (auto to : g[u])
		{
			int v = to.first;
			int w = to.second;
			dis[v] = max(dis[v], dis[u] + w);
			d[v]--;
			if (d[v] == 0)
			{
				q.push(v);
			}
		}
	}
	printf("%d\n", dis[n]);
	return 0;
}