#include <bits/stdc++.h>
#define MAX_N 30001

#define fin cin
#define fout cout
using namespace std;
//ifstream fin("sate.in");
//ofstream fout("sate.out");


struct cv {
    int sat;
    int val;
};

int n, m, x, y;
int ans;
queue<cv> q;
bool sateVazute[MAX_N];

int main() {
    fin >> n >> m >> x >> y;
    sateVazute[x] = true;
    vector<map<int, int>> drumuri(n + 1);
    for (int i = 1; i <= m; i++) {
        int temp_x, temp_y, temp_d;
        fin >> temp_x >> temp_y >> temp_d;
        drumuri[temp_x][temp_y] = temp_d;
        drumuri[temp_y][temp_x] = -temp_d;
    }
    for (const auto &i: drumuri[x]) {
        q.push({i.first, i.second});
        sateVazute[i.first] = true;
    }
    while (!q.empty()) {
        if (q.front().sat == y) {
            ans = q.front().val;
            break;
        }
        for (const auto & i : drumuri[q.front().sat]) {
            if(!sateVazute[i.first]) {
                q.push({i.first,i.second + q.front().val});
                sateVazute[i.first] = true;
            }
        }
        q.pop();
    }
    fout<<ans;
    return 0;
}