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

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

const int nevizitat=300000;

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