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

using namespace std;

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

struct muchie {
    int vecin;
    int deplasare;
};

vector<muchie> g[30001];
int dist[30001];
bool viz[30001];

int main() {
    int n, m, x, y;
    fin>> n >> m >> x >> y;
    for (int c = 1; c<= m; c++ ) {
        int i, j, d;
        fin >> i >> j >> d;
        g[i].push_back({j, d});
        g[j].push_back({i, -d});
    }

    queue<int> q;
    dist[x] = 0; //pornim din nodul x
    viz[x] = true;
    q.push(x); // nodul de start

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

        if (u == y) {
            break;
        }
        for (int vecin = 0; vecin < g[u].size(); vecin++) {
            int v = g[u][vecin].vecin;
            int d = g[u][vecin].deplasare;

            if (!viz[v]) {
                viz[v] = true;
                dist[v] = dist[u] + d;
                q.push(v);
            }
        }
    }
    fout << dist[y] << "\n";
    return 0;
}