#include<stdio.h>
#define maxm 1024*128*2
#define maxn 32*1024
long next[maxm],v[maxm],first[maxn],sel[maxn],Y,t[maxm];

long df(long a,long b)
{
	long x,y;
	if(a==Y) return b;
	sel[a] = 1;
	x= first[a];
	while(x)
	{
		y = v[x];
		if(sel[y]) {x = next[x];continue;}
		long val = t[(x+1)/2],p;
		if(y<a)
			p = df(y,b-val);
		else p = df(y,b+val);
		if(p) return p;
		x = next[x];
	}
	return 0;
}

int main()
{
	long n,m,x,i;
	freopen("sate.in","r",stdin);
	freopen("sate.out","w",stdout);
	scanf("%ld %ld %ld %ld",&n,&m,&x,&Y);
	for(i=1;i<=m;i++)
	{
		scanf("%ld %ld %ld",&v[i*2-1],&v[i*2],&t[i]);
		long a = v[i*2-1],b = v[i*2];
		next[i*2] = first[a];
		next[i*2-1] = first[b];
		first[a] = i *2;
		first[b] = i *2-1;
	}
	printf("%ld\n",df(x,0));
}