#include <string.h>
#include <stdio.h>
#define INF (2000000000);

unsigned int node_cena[12000];
int node_sil[12000];
int node_final[12000];
int node_heap[12000];
struct sil {
  int z;
  int kam;
  int cena;
  int next;
};
struct sil sil[60000];

int heap[52000];
int heap_alloc;

void swap(int at1, int at2)
{
  int x = heap[at1];
  heap[at1] = heap[at2];
  heap[at2] = x;
  node_heap[heap[at1]] = at1;
  node_heap[heap[at2]] = at2;
}

int CENA(int i)
{
  if (i==0)
    return -10;
  if (i>heap_alloc)
    return INF;
  return node_cena[heap[i]];
}

void fixdown(int i)
{
  if (i <= 1)
    return;
  if (CENA(i) > CENA(i/2))
    return;
  swap(i, i/2);
  fixdown(i/2);  
}

#define MIN(a,b) (a<b?a:b)

void
fixup(int i)
{
  int c1, c2, c;
  if (i>heap_alloc)
    return;
  c1 = CENA(2*i);
  c2 = CENA(2*i+1);
  c = MIN(c1, c2);
  if (c < CENA(i)) {
    if (c1 == c) {
      swap(i, 2*i);
      fixup(2*i);
    } else { 
      swap(i, 2*i+1);
      fixup(2*i+1);
    }
  }
}

void insert_heap(int node)
{
  heap[++heap_alloc] = node;
  node_heap[node] = heap_alloc;
  fixdown(heap_alloc);
}

void delete_heap(int node)
{
  int at = node_heap[node];
  swap(heap_alloc, at);
  heap_alloc--;
  node_heap[node] = -1;
  fixup(at);
  fixdown(at);
}

void printheap(void)
{
  int i;
  for (i=1; i<=heap_alloc; i++) {
    printf("%d,%d ", heap[i], CENA(i));
    if (node_heap[heap[i]] != i)
      printf("!!!");
  }
  printf("<- HEAP\n");
}

#define printheap();

int main (int argc, char **argv)
{
  char		temp [65536];

  while (1)
    {
      int N,M,S,C,i,j;
      scanf("%d %d %d %d\n", &N, &M, &S, &C);
      if (!N && !M && !S && !C)
	break;
      for (i=0; i<N; i++) {
	node_cena[i] = INF;
	node_sil[i] = -1;
	node_heap[i] = -1;
	node_final[i] = 0;
      }
      S--; C--;
      for (i=0; i<M; i++) {
	int od, kam, cena;
	scanf("%d %d %d\n", &od, &kam, &cena);
	od--; kam--;
	sil[i].z = od;
	sil[i].kam = kam;
	sil[i].cena = cena;
	sil[i].next = node_sil[od];
	node_sil[od] = i;
      }
      if (S==C) {
	printf("0\n");
	continue;
      }
      node_cena[S] = 0;
      insert_heap(S);
      while(1) {
	printheap();
	i = heap[1];
	node_final[i] = 1;
	if (node_heap[i] != 1) {
	      printf("BLEE3!\n");
	      return 1;
	    }
	delete_heap(i);
	/*	printf("Finalizing: %d/%d\n", i, node_cena[i]);*/
	printheap();
	if (i==C) {
	  printf("%d\n", node_cena[i]);
	  break;
	}
	j = node_sil[i];
	while(j!=-1) {
	  int kam = sil[j].kam;
	  /*	  printf("Examining %d -> %d (%d,%d)\n", sil[j].z, kam, node_cena[kam], sil[j].cena);*/
	  if (node_cena[kam] > node_cena[i] + sil[j].cena) {
	    if (node_final[sil[j].kam]) {
	      printf("BLEE!\n");
	      return 1;
	    }
	    node_cena[kam] = node_cena[i] + sil[j].cena;
	    /*	    printf("Making better, %d cena %d\n", kam, node_cena[kam]);*/
	    if (node_heap[kam] != -1) {
	      delete_heap(kam);
	    }
	    insert_heap(kam);
	    printheap();
	  }
	  j = sil[j].next;
	}
      }
    }

  return 0;
}