#include<bits/stdc++.h>
using namespace std;
using ii = pair<int, int>;

struct edge {
	// Terminal vertex / Weight
	int v, w;
	edge(int v, int w) : v(v), w(w) {}
};

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);

	int n, m, s, t;
	cin >> n >> m >> s >> t;

	// Edges into each vertex.
	vector<vector<edge>> E(n), RE(n);
	map<ii, int> EW;
	while (m--) {
		int u, v, w;
		cin >> u >> v >> w;
		E[u].emplace_back(v, w);
		RE[v].emplace_back(u, w);
		EW[{u, v}] = w;
	}

	// Dijkstra from t.
	vector<int> dist_t(n, -1);
	{
		priority_queue<ii, vector<ii>, greater<ii>> pq;
		pq.push({dist_t[t] = 0, t});
		while (!pq.empty()) {
			int d, u;
			tie(d, u) = pq.top();
			pq.pop();
			if (dist_t[u] != d) continue;
			for (const edge &e : RE[u]) {
				int nd = d + e.w;
				if (dist_t[e.v] < 0 || nd < dist_t[e.v]) {
					dist_t[e.v] = nd;
					pq.push({nd, e.v});
				}
			}
		}
	}

	// For each vertex find its best and worst outgoing edge.
	vector<int> best(n, -1), worst(n, -1);
	for (int i = 0; i < n; ++i) {
		int f = -1, fi = -1, s = -1, si = -1;
		for (const edge &e : E[i]) {
			if (f == -1 || dist_t[e.v]+e.w < f) fi = e.v, f = dist_t[e.v]+e.w;
			if (s == -1 || dist_t[e.v]+e.w > s) si = e.v, s = dist_t[e.v]+e.w;
		}
		if (f >= 0) best[i] = fi;
		if (s >= 0) worst[i] = si;
	}

	int turn = 0, position = s, time = 0;
	// do 3n steps.
	for (int st = 0; st < 3 * n; ++st) {
		if (position == t || best[position] < 0) break;
		if (turn == 0) { // take worst
			time += EW[{position, worst[position]}];
			position = worst[position];
			turn = 1 - turn;
		} else { // take best
			time += EW[{position, best[position]}];
			position = best[position];
			turn = 1 - turn;
		}
	}

	if (position == t)
		cout << time << endl;
	else
		cout << "infinity" << endl;

	return 0;
}