#include<bits/stdc++.h>
using namespace std;
ifstream fin("sate.in");
ofstream fout ("sate.out");
int n,m,x,y;
int ad[301][301],coada[100001],viz[100001];
int dr[100001];
vector<int>gr[100001];
int main(){
    fin>>n>>m>>x>>y;
    for(int i=1;i<=m;i++){
        int a,b,val;
        fin>>a>>b>>val;
        gr[a].push_back(b);
        gr[b].push_back(a);
        ad[a][b]=val;
        ad[b][a]=-val;
    }
    coada[1]=x;
    int p=1;
    int u=1;
    viz[x]=1;
    while(p<=u){
        int nod=coada[p++];
        if(nod==y){
            break;
        }
        for(int i=0;i<gr[nod].size();i++){
            int vecin=gr[nod][i];
            if(viz[vecin]==0){
                coada[++u]=vecin;
                viz[vecin]=1;
                dr[vecin]=ad[nod][vecin]+dr[nod];
            }
        }
    }
    fout<<dr[y];


}