#include      <iostream>
#include      <fstream>
#include      <algorithm>
#include      <vector>
#include      <queue>

using namespace std;

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

int n, m, start, dx[100001], endd, dy[100001], ap[100001];

vector < pair < int, int > > G[100001];
queue  < int > coada;

void bfs(int s, int d[]) {

    bool vizitat[100001];
    for (int i = 1; i <= n; i++) vizitat[i] = false;

    vizitat[s] = true;
    d[s] = 0;
    coada.push(s);

    while (!coada.empty()) {
        int nod_curent = coada.front();
        coada.pop();

        for (auto u : G[nod_curent])
            if (vizitat[u.first] == false) {
                vizitat[u.first] = true;
                coada.push(u.first);
                d[u.first] = d[nod_curent] + u.second;
            }

    }
}

int main() {
    ios::sync_with_stdio(false);
    in.tie(NULL), out.tie(NULL);

    in >> n >> m >> start >> endd;

    for(int i = 1; i <= n; i++) dx[i] = -1, dy[i] = -1;

    for (int i = 1; i <= m; i++) {
        int x, y, dist;
        in >> x >> y >> dist;
        G[x].push_back(make_pair(y, dist));
        G[y].push_back(make_pair(x, -dist));
    }

    bfs(start, dx);
    
    out << dx[endd];

    return 0;

}