#include <iostream>
#include <climits>
#include <algorithm>
#include <queue>

using namespace std;

int main() {
	int n, m, s, t;
    cin >> n >> m >> s >> t;
    vector<vector<pair<int, int>>> out(2*n);
    vector<vector<pair<int, int>>> in(2*n);

    vector<int> dist(2*n, INT_MAX);
    vector<bool> done(2*n, false);
    vector<int> outct(2*n, 0);

    for(int i = 0; i < m; i++) {
        int a, b, w;
        cin >> a >> b >> w;

        out[2*a].push_back(make_pair(2*b+1, w));
        out[2*a+1].push_back(make_pair(2*b, w));
        outct[2*a]++;
        outct[2*a + 1]++;

        in[2*b].push_back(make_pair(2*a+1, w));
        in[2*b+1].push_back(make_pair(2*a, w));
    }

    priority_queue<pair<int, int>, vector<pair<int, int>>, std::greater<pair<int, int>>> q;
    dist[2*t] = 0;
    dist[2*t + 1] = 0;

    q.push(make_pair(0, 2*t));
    q.push(make_pair(0, 2*t + 1));

    while(!q.empty()) {
        auto pair = q.top(); q.pop();
        int cur = pair.second;
        if(done[cur])
            continue;
        done[cur] = true;
        if(cur & 1) {
            dist[cur] = 0;
            for(auto out_pair : out[cur]) {
                int w = out_pair.second;
                int nb = out_pair.first;
                dist[cur] = max(dist[cur], dist[nb] + w);
            }
            for(auto in_pair : in[cur]) {
                int w = in_pair.second;
                int nb = in_pair.first;
                dist[nb] = min(dist[nb], dist[cur] + w);
                q.push(make_pair(dist[nb], nb));
            }
        }
        else {
            for(auto in_pair : in[cur]) {
                int nb = in_pair.first;
                outct[nb]--;
                if(outct[nb] == 0) {
                    q.push(make_pair(0, nb));
                }
            }
        }
    }

    if(dist[2*s + 1] == INT_MAX)
        cout << "infinity" << endl;
    else
        cout << dist[2*s+1] << endl;

	return 0;
}