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

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

int n, m, a, b, dist[30005];
vector<pair<int, int>>v[30005];
queue<int>q;

void bfs(){
    while(!q.empty()){
        int nod = q.front();
        q.pop();
        if(nod==b){
            fout << dist[nod]-1;
            return ;
        }
        for(auto y : v[nod]){
            if(dist[y.first]==0){
                if(y.first < nod){
                    dist[y.first]=dist[nod]+y.second;
                    q.push(y.first);
                }
                else{
                    dist[y.first]=dist[nod]-y.second;
                    q.push(y.first);
                }
            }
        }
    }
}

int main(){
    fin >> n >> m >> a >> b;
    for(int i = 1; i <= m; ++i){
        int x, y, c;
        fin >> x >> y >> c;
        v[x].push_back(make_pair(y, c));
        v[y].push_back(make_pair(x, c));
    }
    dist[a] = 1;
    q.push(a);
    bfs();
    return 0;
}