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

const int N = 1e3 + 5;
const double vc = (double)(1e15);
const double eps = 1e-3;

int n, m;
vector < pair < int , int > > adj[N];
double d[N];
bool flag = false;
int visit[N];

void dfs(int u) {
	visit[u] = 1;
	for (auto it : adj[u]) {
		int v = it.first;
		if (visit[v] == 1) flag = true;
		if (visit[v] == 0) dfs(v);
	}
	visit[u] = 2;
}

bool cycle() {
	for (int i = 1; i <= n; i++)
		if (!visit[i]) dfs (i);
	return flag;
}

bool shimbel(double val) {
	for (int i = 1; i <= n; i++) d[i] = vc;
	d[1] = 0;

	for (int i = 1; i < n; i++)
		for (int u = 1; u <= n; u++)
			for (auto it : adj[u]) {
				int v = it.first, w = it.second;
				if (d[u] > d[v] + w - val) {
					d[u] = d[v] + w - val;
				}
			}

	for (int u = 1; u <= n; u++)
		for (auto it : adj[u]) {
			int v = it.first, w = it.second;
			if (d[u] > d[v] + w - val)
				return false;
		}

	return true;
}

int main() {
	scanf ("%d %d", &n, &m);
	for (int i = 0; i < m; i++) {
		int u, v, w;
		scanf ("%d %d %d", &u, &v, &w);
		adj[u].push_back({v, w});
	}

	if (!cycle()) {
		printf("-1\n");
		return 0;
	}

	double d = 0, c = vc, res = -1;
	while (d <= c) {
		double g = (d + c) / 2;
		if (shimbel(g)) {
			res = g;
			d = g + eps;
		} else c = g - eps;
	}

	printf("%.2f\n", res);

	return 0;
}