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

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

int n, m, x, y;
long long poz[30005];
bool vizitat[30005];
vector<pair<int, int>> adj[30005];

int main() {
    fin >> n >> m >> x >> y;

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

    queue<int> q;
    q.push(x);
    vizitat[x] = true;
    poz[x] = 0;

    while (!q.empty()) {
        int u = q.front();
        q.pop();

        for (auto e : adj[u]) {
            int w = e.first;
            if (!vizitat[w]) {
                vizitat[w] = true;
                poz[w] = poz[u] + e.second;
                q.push(w);
            }
        }
    }

    fout << llabs(poz[y]) << '\n';
    return 0;
}