#include <stdio.h>
#include <stdlib.h>
#define infile "sate.in"
#define outfile "sate.out"
#define nmax 30000

int n, *v[nmax], x, y, i, c2[2][nmax];
long m, *c[nmax], dist[nmax];

void readdata()
{
 FILE *fin=fopen(infile, "r");
 int x2, y2;
 long c2;
 fscanf(fin, "%d %ld %d %d\n", &n, &m, &x, &y);
 for (i=1; i<=n; i++)
     {
      v[i]=(int *)realloc(v[i], sizeof(int));
      v[i][0]=0;
      c[i]=(long *)realloc(c[i], sizeof(long));
      c[i][0]=0;
     }
 for (i=0; i<m; i++)
     {
      fscanf(fin, "%d %d %d\n", &x2, &y2, &c2);
      v[x2][0]++;
      v[x2]=(int *)realloc(v[x2], (v[x2][0]+1)*sizeof(int));
      v[x2][v[x2][0]]=y2;
      c[x2][0]++;
      c[x2]=(long *)realloc(c[x2], (c[x2][0]+1)*sizeof(long));
      c[x2][c[x2][0]]=c2;
      v[y2][0]++;
      v[y2]=(int *)realloc(v[y2], (v[y2][0]+1)*sizeof(int));
      v[y2][v[y2][0]]=x2;
      c[y2][0]++;
      c[y2]=(long *)realloc(c[y2], (c[y2][0]+1)*sizeof(long));
      c[y2][c[y2][0]]=c2;
     }
}

void solve(int x)
{
 int k=1, u2=1, u=0, p=0;
 c2[0][u2]=x;
 dist=(long *)realloc(dist, (n+1)*sizeof(long));
 for (i=1; i<=n; i++) dist[i]=0;
 while (u2)
     {
      u=u2;
      p=1;
      k=1-k;
      u2=0;
      while (p<=u)
	  {
	   x=c2[k][p++];
	   for (i=1; i<=v[x][0]; i++)
	       if (!dist[v[x][i]])
		   {
		    c2[1-k][++u2]=v[x][i];
		    if (x<v[x][i])
			dist[v[x][i]]=dist[x]+c[x][i];
		    else
			dist[v[x][i]]=dist[x]-c[x][i];
		    if (dist[y]) return;
		   }
	  }
     }
}

void writedata()
{
 FILE *fout=fopen(outfile, "w");
 fprintf(fout, "%ld", dist[y]>0?dist[y]:-dist[y]);
 fclose(fout);
}

int main()
{
 readdata();
 solve(x);
 writedata();
 return 0;
}