#include<bits/stdc++.h>
using namespace std;
ifstream fin("sate.in");
ofstream fout ("sate.out");

int n,m,x,y;
int coada[100001],viz[100001];
int dr[100001];
vector<pair<int,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,val});
        gr[b].push_back({a,-val});
    }

    coada[1]=x;
    viz[x]=1;
    int p=1;
    int u=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].first;
            int cost=gr[nod][i].second;

            if(viz[vecin]==0){
                coada[++u]=vecin;
                viz[vecin]=1;
                dr[vecin]=dr[nod]+cost;
            }
        }
    }

    fout<<dr[y];
}