#include <array>
#include <iostream>
#include <queue>
#include <vector>
using namespace std;
using ll = int;
// Dijkstra
struct Edge{ int v; ll weight; };	// input edges
struct S{ int u; bool b; };
struct PQ{ ll d; S v; };			// distance and target
bool operator>(const PQ &l, const PQ &r){ return l.d > r.d; }
ll dijkstra(const vector<vector<Edge>> &edges, const vector<int>& deg, int s, S t) {
	array<vector<ll>,2> dist;
	vector<ll> counts(edges.size());
	dist[0].assign(edges.size(), 1e18);
	dist[1].assign(edges.size(), 0);
	priority_queue<PQ,vector<PQ>,greater<PQ>> pq;
	dist[0][s] = dist[1][s] = 0;
	pq.push({0, {s, true}}); pq.push({0 ,{s, false}});
	while (!pq.empty()) {
		ll d = pq.top().d; int u = pq.top().v.u; bool b = pq.top().v.b; pq.pop();
		if(u == t.u && b == t.b) return d;
		if(d != dist[b][u]) continue;
		if(b) {
			for(auto &e : edges[u])
				if (d + e.weight < dist[0][e.v])
					pq.push({dist[0][e.v] = d + e.weight, {e.v, !b}});
		} else {
			for(auto &e : edges[u]){
				dist[1][e.v] = max(dist[1][e.v], d + e.weight);
				if(++counts[e.v] == deg[e.v])
					pq.push({dist[1][e.v], {e.v, !b}});
			}
		}
	}
	return -1;
}
int main(){
	int n, m, s, t;
	cin >> n >> m >> s >>t;
	vector<vector<Edge>> g(n);
	vector<int> deg(n);
	while(m--){
		int u,v,w; cin >> u >> v >> w;
		g[v].push_back({u,w});
		++deg[u];
	}
	auto d = dijkstra(g, deg, t, {s,true});	
	if(d >= 0) cout << d << endl;
	else cout << "infinity\n";
}