#include <bits/stdc++.h>
#define pb push_back
#define mp make_pair
#define sz size()
using namespace std;
const int NR = 30001 ;
ifstream f("sate.in");
ofstream g("sate.out");
vector < pair < int , int > > v [ NR ] ;
int d [ NR ] , stop ;

int bfs ( int q ){
    int prim = 1 , ultim = 1 , coada[ NR ] ;
    coada [ 1 ] = q ;
    d [ q ] = 1 ;
    while ( prim <= ultim ){
        int nod = coada [ prim ] ;
        if ( nod == stop )  return g << abs ( d [ stop ] - 1 ) << "\n" , 0 ;
        for ( size_t i = 0 ; i < v [ nod ].sz ; ++ i ){
            int vecin = v [ nod ][ i ].first ;
            int cost = v [ nod ][ i ].second ;
            if ( !d[ vecin ] )
            {
                if ( vecin > nod )  d[ vecin ] = d [ nod ] + cost ;
                else                d[ vecin ] = d [ nod ] - cost ;
                coada[ ++ ultim ] = vecin ;
            }}prim ++ ;}}
int main (){
    int n , m , start ;
    f >> n >> m >> start >> stop ;
    while ( m -- ){
        int a , b , c ; f >> a >> b >> c ;
        v [ a ].pb( mp ( b , c ) ) ;
        v [ b ].pb( mp ( a , c ) ) ;}
    bfs ( start ) ;}