#include <iostream>
#include <fstream>
#include <vector>
#include <queue>
#define inf 1<<30
#define nmax 30005

using namespace std;

int n, m, x, y, dist[nmax], viz[nmax];
vector <pair <int, int> > G[nmax];
queue <int> Q;

void read(){
    int n1, n2, d;
    ifstream f("sate.in");
    f >> n >> m >> x >> y;
    for(int i=1; i<=m; i++){
        f >> n1 >> n2 >> d;
        G[n1].push_back(make_pair(n2, d));
        G[n2].push_back(make_pair(n1, d));
    }
}

void setdist(int st){
    for(int i=1; i<=n; i++)
        dist[i] = -1,
        viz[i] = 0;
    dist[st] = 0;
    viz[st] = 1;
    Q.push(st);
}

void dijkstra(){
    int nod = 0, v = 0, w = 0;
    while(!Q.empty()){
        nod = Q.front();
        Q.pop();
        for(int i=0; i<G[nod].size(); i++){
            v = G[nod][i].first;
            w = G[nod][i].second;
            if(dist[v] == -1){
                if(nod < v)
                    dist[v] = dist[nod] + w;
                else
                    dist[v] = dist[nod] - w;
                viz[v] = 1;
                Q.push(v);
            }
        }
    }
}

int main()
{
    read();
    setdist(x);
    dijkstra();
    ofstream g("sate.out");
    g << dist[y];
    return 0;
}