#include <stdio.h>
#include <vector>
using namespace std;

const int DIM = 100005;
int N, M, X, Y, C[DIM], D[DIM], viz[DIM];

vector <int> V[DIM], L[DIM];

int main ()
{
	freopen ("sate.in", "r", stdin);
	freopen ("sate.out", "w", stdout);
	
	scanf ("%d%d%d%d", &N, &M, &X, &Y);
	for (int i = 0, a, b, d; i < M; i++)
	{
		scanf ("%d%d%d", &a, &b, &d);
		V[a].push_back (b);
		V[b].push_back (a);
		if (a < b)
			L[a].push_back (d),
			L[b].push_back (-d);
		else
			L[a].push_back (-d),
			L[b].push_back (d);
	}
	
	int p = 0, u = 0;
	C[0] = X;
	D[0] = 0;
	viz[X] = 1;
	while (p <= u)
	{
		for (int i = 0; i < V[C[p]].size(); i++)
			if (!viz[V[C[p]][i]])
			{
				u++;
				C[u] = V[C[p]][i];
				D[V[C[p]][i]] = D[C[p]] + L[C[p]][i];
				viz[V[C[p]][i]] = 1;
				if (V[C[p]][i] == Y)
					i = V[C[p]].size(), p = u + 1;
			}		
		p++;
	}
	
	printf ("%d", D[Y]);
	return 0;
}