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

struct nod{

  int nodv,val;

}temp;

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

 vector <vector < nod > > graf;
 vector < int > Cost;
 queue < int > q;
 int n,m,x,y,xs,ys,c;

void citire(){

    f >> n >> m >> x >> y;

graf.resize(n+1);
 Cost.resize(n+1);

    for(int i=1;i<=m;i++){
        f >> xs >> ys >> c;
        temp.nodv = ys; temp.val = c;
        graf[xs].push_back(temp);
        temp.nodv = xs; temp.val = -c;
        graf[ys].push_back(temp);
    }
 }

void Dijkstra(){

    int first = x;
  q.push(x);
        while(!q.empty()){
        for(vector<nod>::iterator it = graf[first].begin();it!=graf[first].end();++it){
            if(Cost[it->nodv] == 0){
                Cost[it->nodv] = Cost[first] + it->val;
                q.push(it->nodv);
            }
            if(it->nodv == y){
                g << Cost[y];
                return;
        }}
        q.pop();
        first = q.front();
        }
}

int main(){

    citire();
    Dijkstra();

f.close();
g.close();
return 0;
}