#include <bits/stdc++.h>
using namespace std;

int n, m, x, y;
vector<vector<pair<int, int>>> v(30001);
int cost[30001];

void bfs() {
    memset(cost, -1, sizeof(cost));
    queue<int> q;
    q.push(x);
    cost[x] = 0;
    while (!q.empty()) {
        int node = q.front();
        if (node == y) {
            return;
        }
        q.pop();
        for (int i = 0; i < v[node].size(); ++i) {
            if (cost[v[node][i].first] == -1) {
                cost[v[node][i].first] = cost[node] + v[node][i].second;
                q.push(v[node][i].first);
            }
        }
    }
}

int main() {
    ifstream cin("sate.in");
    ofstream cout("sate.out");
    cin >> n >> m >> x >> y;
    for (int i = 1; i <= m; ++i) {
        int a, b, c;
        cin >> a >> b >> c;
        v[a].push_back({b, c});
        v[b].push_back({a, -c});
    }
    bfs();
    cout << cost[y];
}