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

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

int n, m, X, Y;
vector<pair<int,int>> g[30001];
int dist[30001];
bool viz[30001];

int main() {
    fin >> n >> m >> X >> Y;
    for (int k = 0; k < m; k++) {
        int i, j, d;
        fin >> i >> j >> d;
        if (i > j) swap(i, j);
        g[i].push_back({j,  d});
        g[j].push_back({i, -d});
    }
    queue<int> q;
    q.push(X); viz[X] = true; dist[X] = 0;
    while (!q.empty()) {
        int u = q.front(); q.pop();
        if (u == Y) break;
        for (auto [v, c] : g[u])
            if (!viz[v]) {
                viz[v] = true;
                dist[v] = dist[u] + c;
                q.push(v);
            }
    }
    int r = dist[Y];
    fout << (r < 0 ? -r : r);
    return 0;
}