#include <iostream>
#include <fstream>
#include <vector>
#include <queue>

using namespace std;

ifstream f("sate.in");
ofstream g("sate.out");

int n, m, x, y;

int main() {
    f >> n >> m >> x >> y;

    vector<vector<pair<int, int>>> lista(n + 1);
    for (int i = 0; i < m; i++) {
        int u, v, dist;
        f >> u >> v >> dist;
        lista[u].push_back({v, dist});
        lista[v].push_back({u, -dist});
    }

    vector<bool> viz(n + 1, false);
    vector<int> d(n + 1, 0);
    queue<int> coada;

    coada.push(x);
    viz[x] = true;
    d[x] = 0;

    while (!coada.empty()) {
        int nod = coada.front();
        coada.pop();

        if (nod == y) {
            break;
        }

        for (int i = 0; i < lista[nod].size(); i++) {
            int vecin = lista[nod][i].first;
            int cost = lista[nod][i].second;

            if (!viz[vecin]) {
                viz[vecin] = true;
                d[vecin] = d[nod] + cost;
                coada.push(vecin);
            }
        }
    }

    g<<d[y]-d[x] << "\n";

    return 0;
}