#include <iostream>
#include <fstream>
#include <vector>
#include <queue>
using namespace std;

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

const int MAX=30001;
int nevizitat=10000000;

int main(){
    int n,m,x,y,a,b,c;
    in>>n>>m>>x>>y;
    vector<vector<pair<int,int>>>graf(n+1);
    vector<int>dist(n+1,nevizitat);
    queue<int>q;
    for(int i=1;i<=m;i++){
        in>>a>>b>>c;
        graf[a].push_back({b,c});
        graf[b].push_back({a,c});
    }
    dist[x]=0;
    q.push(x);
    while(!q.empty()){
        int u=q.front();
        q.pop();
        for(auto vecin : graf[u]){
            int v=vecin.first;
            int dis=vecin.second;
            if(dist[v]==nevizitat){
                if(u<v){
                    dist[v]=dist[u]+dis;
                }else if(u>v){
                    dist[v]=dist[u]-dis;
                }
                q.push(v);
            }
        }
    }
    out<<dist[y];
}