#include <stdio.h>
#include <list>
#define NRSATE 30001
using namespace std;

struct rel {
	long dest;
	long dist;
};

char parcurs[NRSATE];
list<rel> sat[NRSATE];

long parg_rez=0;
void parg (long x, long y, long cdis) {
      parcurs[x]=1;
//      fprintf(stderr,"parg(x==%ld,y==%ld,cdis==%ld)\n",x,y,cdis);
	if (x==y) { parg_rez=cdis; } 
	else {
	      for (list<rel>::iterator it=sat[x].begin(); it!=sat[x].end(); it++) {
		    if ((parg_rez==0) && (!parcurs[it->dest])) parg(it->dest,y,cdis+it->dist);
	      };
	};
};
int main (void) {
      	FILE * fi = fopen("sate.in","rt");
	FILE * fo = fopen("sate.out","wt");

	long n,m,x,y;
	fscanf(fi,"%ld %ld %ld %ld",&n,&m,&x,&y);
	for (long i=1; i<=m; i++) {
	      	long sor,sdest,sdist;
		fscanf(fi,"%ld %ld %ld",&sor,&sdest,&sdist);
		rel tmprf;
		tmprf.dest=sdest;
		tmprf.dist=sdist;
		sat[sor].push_back(tmprf);
		tmprf.dest=sor;
		tmprf.dist=-sdist;
		sat[sdest].push_back(tmprf);
	};
	parg(x,y,0);
	fprintf(fo,"%ld",parg_rez);

	fclose(fi); fclose(fo);
	return 0;
};