#include<fstream>
#include<vector>
#include<queue>
#define NMAX 30005
 
using namespace std;
 
ifstream f("sate.in");
ofstream g("sate.out");
 
int n,m,x,y,q,w,d,viz[NMAX],dist[NMAX],i,nod;
 
vector< pair <int,int > >G[NMAX];
queue<int>Q;
int main (){
 
    f>>n>>m>>x>>y;
     
    for(i=1;i<=m;i++){
         
           f>>q>>w>>d;
           if(q>w)
               swap(q,w);
        G[q].push_back(make_pair(w,d));
        G[w].push_back(make_pair(q,-d));
    }
     
    viz[x]=1;
    Q.push(x);
    bool ok=1;
    while(!Q.empty() && ok){
         
        nod=Q.front();
        Q.pop();
        for(int i=0;i<G[nod].size();++i){
             
            if(!viz[G[nod][i].first]){
                 
                viz[G[nod][i].first]=1;
                dist[G[nod][i].first]+=dist[nod]+G[nod][i].second;
                Q.push(G[nod][i].first);
                 
            }
             
        }
    }
    g<<dist[y];
    return 0;
}