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

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

struct graf
{
    int nod, cost;
};

int n, m, i, j, x, X, Y, p[30002];
vector <graf> a[30002];
bool v[30002];

void BFS(int nod) {
    queue <int> q;
    q.push(nod);
    p[X] = 0;
    v[nod] = 1;
    while (!q.empty()) {
        int x = q.front();
        q.pop();
        for (graf it : a[x]) {
            if (!v[it.nod]) {
                p[it.nod] = p[x] + it.cost;
                q.push(it.nod);
                v[it.nod] = 1;
            }
        }
        if (x == Y)
            g << p[Y], exit(0);
    }
}

int main()
{
    f >> n >> m >> X >> Y;
    while (m--) {
        f >> i >> j >> x;
        a[i].push_back({j, x});
        a[j].push_back({i, -x});
    }
    BFS(X);
    g << p[Y] - p[X];

    return 0;
}