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

using namespace std;

const int NMAX = 30001;

vector < pair<int, int> > a[NMAX];
vector <int> d(NMAX, -1);

void bfs(int s){
    queue < pair<int, int> > q;
    d[s] = 0;
    q.push(make_pair(s, 0));

    while(!q.empty()){
        pair <int, int> x = q.front();
        q.pop();
        for(auto y : a[x.first]){
            if(d[y.first] == -1){
                d[y.first] = y.second + d[x.first];
                q.push(y);
            }
        }
    }

}

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

    int n, m, x, y;
    fin >> n >> m >> x >> y;

    for(int i = 1; i <= m; i++){
        int u, v, d;
        fin >> u >> v >> d;
        a[u].push_back(make_pair(v, d));
        a[v].push_back(make_pair(u, -d));
    }

    bfs(x);

    fout << d[y];

    return 0;
}