#include <bits/stdc++.h>
using namespace std;
ifstream in("sate.in");
ofstream out("sate.out");

const int INF = 1e9;

int main() {
    int n, m, x, y;
    in >> n >> m >> x >> y;
    vector<vector<pair<int, int>>> adj(n + 1);

    for (int i = 1; i <= m; i++) {
        int a, b, c;
        in >> a >> b >> c;
        adj[a].push_back({b, c});
        adj[b].push_back({a, -c});
    }
    vector<int> dist(n + 1, INF);
    queue<int> q;

    dist[x] = 0;
    q.push(x);
    while (!q.empty()) {
        int u = q.front();
        q.pop();
        if (u == y) break;
        for (auto muchie : adj[u]) {
            int v = muchie.first;
            int cost = muchie.second;
            if (dist[v] == INF) {
                dist[v] = dist[u] + cost;
                q.push(v);
            }
        }
    }

    out << dist[y] << "\n";

    return 0;
}