#include<bits/stdc++.h>
using namespace std;

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

struct state {
	// Brain half / Vertex
	int h, v;
	state(int h, int v) : h(h), v(v) {}
	state() : h(0), v(0) {}
};

class compare_wstate {
  public:
	bool operator() (const pair<int, state> &l, const pair<int, state> &r) {
		return l.first > r.first;
	}
};

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

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

	// Reverse edges into each vertex.
	vector<vector<edge>> reverse(n);
	vector<int> adj(n, 0);
	while (m--) {
		int u, v, w;
		cin >> u >> v >> w;
		reverse[v].emplace_back(u, w);
		adj[u]++;
	}

	priority_queue<
		pair<int, state>,
		vector<pair<int, state>>,
		compare_wstate> pq;
	vector<vector<int>> d(2, vector<int>(n, -1));
	// We want to reach {t, left half turn} and reason backwards,
	// Dijkstra-style.
	pq.push({d[0][t] = 0, state{0, t}});
	pq.push({d[1][t] = 0, state{1, t}});
	while (!pq.empty()) {
		int dst;
		state st;
		tie(dst, st) = pq.top();
		pq.pop();

		if (d[st.h][st.v] != dst) continue;

		for (const edge &e : reverse[st.v]) {
			int nd = dst + e.w;
			// Are we forced to move from e.v in state !st.h ?
			if (st.h == 0) { // always move, we are in st.h = 1 in e.v
				if (d[1][e.v] >= 0 && d[1][e.v] <= nd)
					continue;
				d[1][e.v] = nd;
				pq.push({nd, state{1, e.v}});
			} else { // only move if forced, we are in st.h = 0 in e.v
				adj[e.v]--;
				d[0][e.v] = max(nd, d[0][e.v]);
				if (adj[e.v] == 0) {
					pq.push({d[0][e.v], state{0, e.v}});
				}
			}
		}
	}

	// We start in {0, s}.
	if (d[0][s] < 0 || adj[s] > 0)
		cout << "infinity" << endl;
	else
		cout << d[0][s] << endl;

	return 0;
}