#include<fstream>
#include<iostream>
using namespace std;
#define nn 30005
struct nod
{
	int info,dist;
	nod *next;
};
nod *g[nn];
int n,m,x,y,D[nn];
void adauga(int a,int b,int d)
{
	nod *t=new nod;
	t->info=b;
	t->dist=d;
	t->next=g[a];
	g[a]=t;
}
int bfs()
{
	int coada[nn];
	int v[nn];
	int st,dr;
	for(int i=1;i<=n;++i)
		v[i]=0;
	
	st=dr=1;
	coada[st]=x;
	v[1]=1;	
	while(coada[st]!=y&&st<=dr)
	{
		for(nod *t=g[coada[st]];t;t=t->next)
			if(!v[t->info])
			{
				coada[++dr]=t->info;
				v[t->info]=1;
				if(coada[st]<coada[dr])
					D[dr]=D[st]+g[coada[st]]->dist;
				else D[dr]=D[st]-g[coada[st]]->dist;
			}
			else
				g[coada[st]]=g[coada[st]]->next;
		st++;
		cout<<st<<" "<<coada[st]<<" "<<D[st]<<" ";
		cin.get();
	}
	
	return D[st];
//	for(int i=1;i<=n;++i)
	//	cout<<coada[i]<<" ";
}
int main()
{
	int i,j,d;
	ifstream fin("sate.in");
	fin>>n>>m>>x>>y;
	for(;m;--m)
	{
		fin>>i>>j>>d;
		adauga(i,j,d);
		adauga(j,i,d);
	}
	/*for(int i=1;i<=n;++i)
	{
		cout<<i<<": ";
		for(nod *t=g[i];t;t=t->next)
		{
			cout<<t->info<<"("<<t->dist<<")"<<" ";
		}
		cout<<endl;
	}
	*/
	d=bfs();
	FILE *f=fopen("sate.out","w");
	fprintf(f,"%d",d);
	return 0;
}