#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,u,c[10025],i,j,m,x,y,h;

void bf(int s) {
	int p,u,i,j;
	p=u=1; c[1]=s;
	viz[s]=1; d[s]=0;
	while(p<=u) {
		i=c[p];
		p=p+1;
		for(j=1;j<=n;j++)
			if(a[i][j] && viz[j]==0){
				u=u+1;
				c[u]=j;
				viz[j]=1;
				if(j>i)
				 d[j]=d[i]+a[i][j];
				 else
                 d[j]=d[i]-a[i][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]=h; }
bf(x);
g<<d[y];
}