#include<fstream>
#include <vector>
#include <bitset>
#include <queue>

std::ifstream fin("sate.in");
std::ofstream fout("sate.out");

int n, m, X, Y;
std::queue<std::pair<int, int>> q;
std::bitset<30005> viz;
std::vector<std::pair<int, int>> L[30005];

int main() {
    int a, b, d;
    fin >> n >> m >> X >> Y;

    for (int i = 1; i <= m; i++) {
        fin >> a >> b >> d;
        L[a].push_back(std::make_pair(b, d));
        L[b].push_back(std::make_pair(a, -d));
    }

    q.push(std::make_pair(X, 0));
    viz[X] = 1;
    while (!q.empty()) {
        auto[k, d] = q.front();
        // k = q.front().first
        // d = q.front().second
        q.pop();

        for (auto& w : L[k]) {
            if (viz[w.first] == 0) {
                q.push(std::make_pair(w.first, d + w.second));
                viz[w.first] = 1;

                if (w.first == Y) {
                    fout << d + w.second << "\n";
                    return 0;
                }
            }
        }
    }


}