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

using namespace std;

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

const int MAXN = 30005;
vector<pair<int, int>> adj[MAXN];
int dist[MAXN];
bool viz[MAXN];

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

    for (int i = 0; i < m; ++i) {
        int u, v, d;
        fin >> u >> v >> d;
        if (u > v) swap(u, v);
        adj[u].push_back({v, d});
        adj[v].push_back({u, -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) {
            fout << dist[y] << "\n";
            return 0;
        }

        for (auto& edge : adj[u]) {
            int v = edge.first;
            int cost = edge.second;
            if (!viz[v]) {
                viz[v] = true;
                dist[v] = dist[u] + cost;
                q.push(v);
            }
        }
    }

    return 0;
}