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

using namespace std;

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

int n, x, y;
struct muchie {
    int nod, cost;
    muchie(int nod, int cost) {
        this->nod = nod;
        this->cost = cost;
    }
};
vector <muchie> v[30001];
int d[30001];

void citeste () {
    int m, a, b, c;
    f >> n >> m >> x >> y;
    for (int i = 1; i <= m; i++) f >> a >> b >> c, v[a].push_back(muchie(b, c)), v[b].push_back(muchie(a, c));
}

void DFS (int nod) {
    if (nod != y) {
        int l = v[nod].size();
        for (int i = 0; i < l; i++)
            if (d[v[nod][i].nod] == -1) {
                if (v[nod][i].nod < nod) d[v[nod][i].nod] = d[nod] - v[nod][i].cost;
                else d[v[nod][i].nod] = d[nod] + v[nod][i].cost;
                DFS(v[nod][i].nod);
            }
    }
}

int main () {
    citeste ();
    for (int i = 2; i <= n; i++) d[i] = -1;
    DFS (x);
    g << d[y] << '\n';
    return 0;
}