#include<stdio.h>
#include<vector>

int sel[ 32 * 1024], sol[ 32 * 1024];
using namespace std;

vector < vector < int > > muchii;
vector < vector < int > > cost;

void df( int poz, int end, int n, int dist)
{
	sel[poz] = 1;
	sol[poz] = dist;
	if( poz == end) return ;
	for( int i = 0; i < muchii[poz].size(); ++i)
	{
		if( sel[ muchii[poz][i] ] == 0)
		{
			df( muchii[poz][i], end, n, cost[poz][i] + dist); 
		}
	}
}
int main()
{
	freopen("sate.in","r",stdin);
	freopen("sate.out","w",stdout);

	vector < int > temp;
	
	int n,m,x,y,i;
	scanf("%d %d %d %d",&n,&m,&x,&y);
	for( i = 0; i <= n; ++i)
		muchii.push_back(temp), cost.push_back(temp);

	for( i = 1; i <= m; ++i)
	{
		int a,b,c;
		scanf("%d %d %d",&a,&b,&c);
		muchii[a].push_back(b);
		muchii[b].push_back(a);
		cost[a].push_back(c);
		cost[b].push_back(-c);
	}
	df(x,y,n,0);

	printf("%d\n", sol[y]);


	return 0;
}