#include<bits/stdc++.h>
using namespace std;
int n,m,x,y,z,o1,o2;
long long rs[30005];
vector< pair<int,int> > v[30005];bool viz[30005];
queue<int> q;
void bfs(int nod){
    q.push(nod);
    viz[nod]=1;
    while(!q.empty()){
        int nod=q.front();
        q.pop();
        viz[nod]=1;
        for(int i=0;i<v[nod].size();i++)
        if(!viz[v[nod][i].first]){
            q.push(v[nod][i].first);
            rs[v[nod][i].first]+=rs[nod]+v[nod][i].second;
            if(v[nod][i].first==o2)return;
        }
    }
}
int main(){
    ifstream cin("sate.in");
    ofstream cout("sate.out");
    cin>>n>>m>>o1>>o2;
    for(int i=1;i<=m;i++){
        cin>>x>>y>>z;
        v[y].push_back(make_pair(x,-z));
        v[x].push_back(make_pair(y,z));
    }
    bfs(o1);
    cout<<rs[o2];
    return 0;
}