#include<iostream>
#include<fstream>
using namespace std;
ifstream f("sate.in");
ofstream g("sate.out");
int n, a[10025][10025], viz[10025], d[10025],p,i,j,m,x,y,h;

void df(int i) {
	viz[i]=1;
    for(int j=1;j<=n;j++)
        if(a[i][j] && viz[j]==0) {
            if(i<j) d[j]=d[i]+a[i][j];
             else d[j]=d[i]-a[i][j];
            df(j);
        } }
int main()
{
f>>n>>m>>x>>y;
for(p=1;p<=m;p++){
	f>>i>>j>>h;
	a[i][j]=h;
	 a[j][i]=a[i][j]; }
df(x);
g<<d[y];
}