#include <stdio.h>
#include <string.h>

const int NMAX=30010;
const long MMAX=100100;

int x[MMAX], y[MMAX], n, ax, ay, *a[NMAX], q[NMAX], cont[NMAX];
long tc[MMAX], m, *c[NMAX], dist[NMAX];

long bf(int s, int d);

int main()
{
	long i;
	freopen("sate.in", "r", stdin);
	freopen("sate.out", "w", stdout);
	scanf("%d%ld%d%d", &n, &m, &ax, &ay);
	for (i=0; i<m; i++)
	{
		scanf("%d%d%ld", &x[i], &y[i], &tc[i]);
		cont[x[i]]++;
		cont[y[i]]++;
	}//for i
	for (i=1; i<=n; i++)
	{
		a[i]=new int[cont[i]+1];
		c[i]=new long[cont[i]+1];
		a[i][0]=0;
		c[i][0]=0;
	}//for i
	for (i=0; i<m; i++)
	{
		a[x[i]][++a[x[i]][0]]=y[i];
		a[y[i]][++a[y[i]][0]]=x[i];
		c[x[i]][++c[x[i]][0]]=tc[i];
		c[y[i]][++c[y[i]][0]]=-tc[i];		
	}//for i
	printf("%ld", bf(ax, ay));
	return 0;
}//main

long bf(int s, int d)
{
	int u=0, p=0, t, i;
	memset(dist, -1, sizeof(dist));
	q[p]=s;
	dist[s]=0;
	while (p<=u)
	{
		t=q[p++];
		for (i=1; i<=a[t][0]; i++)
			if (dist[a[t][i]]==-1)
			{
				q[++u]=a[t][i];
				dist[a[t][i]]=dist[t]+c[t][i];
				if (a[t][i]==d)
					return dist[a[t][i]];
			}//for i
	}//while
	return 0;
}//bf