#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 dfs ( int nod ){
    if ( nod == stop )  return g << abs ( d [ nod ] - 1 ) << "\n" , 0 ;
    for ( size_t i = 0 ; i < v [ nod ].sz ; ++ i ){
        int vecin = v [ nod ][ i ].first ;
        int dist = v [ nod ][ i ].second ;
        if ( !d [ vecin ] ){
            if ( vecin > nod )  d [ vecin ] = dist + d [ nod ] ;
            else                d [ vecin ] = d [ nod ] - dist ;
            dfs ( vecin ) ;}}}
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 ) ) ;}
    d [ start ] = 1 ;
    dfs ( start ) ;}