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

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

std::vector<std::pair<int, int>> v[30001];
int dist[30001];
bool viz[30001];

int main() {
    int n, m, x, y;
    fin >> n >> m >> x >> y;
    
    int a, b, c;
    for (int i = 1; i <= m; i++) {
        fin >> a >> b >> c;
        v[a].push_back({b, c});
        v[b].push_back({a, -c});
    }
    std::queue<int> q;
    dist[x] = 0;
    viz[x] = true;
    q.push(x);

    while (!q.empty()) {
        int nod = q.front();
        q.pop();
        for (auto per : v[nod]) {
            int vecin = per.first;
            int cost = per.second;
            if (!viz[vecin]) {
                dist[vecin] = dist[nod] + cost; 
                viz[vecin] = true;
                q.push(vecin);
            }
            if (vecin == y) {
                fout << abs(dist[y]);
                return 0;
            }
        }
    }

    return 0;
}