#include <fstream>
#include <vector>
#include <queue>
#include <cstdlib>
using namespace std;

int main() {
    ifstream fin("sate.in");
    ofstream fout("sate.out");

    int n, m, x, y;
    fin >> n >> m >> x >> y;

    vector<vector<pair<int,int>>> g(n + 1);
    for (int k = 0; k < m; k++) {
        int i, j, d;
        fin >> i >> j >> d;
        g[i].push_back({j, d});
        g[j].push_back({i, -d});
    }

    vector<int> poz(n + 1, 0);
    vector<bool> vis(n + 1, false);
    queue<int> q;
    q.push(x);
    vis[x] = true;  

    while (!q.empty() && !vis[y]) {
        int u = q.front(); q.pop();
        for (auto [v, c] : g[u]) {
            if (!vis[v]) {
                vis[v] = true;
                poz[v] = poz[u] + c;
                q.push(v);
            }
        }
    }

    fout << abs(poz[y]) << "\n";
    return 0;
}