public static void main ( String [ ] $ ) {
  Scanner sc = new Scanner ( in ) ;
  int N = sc . nextInt ( ) ;
  int a = sc . nextInt ( ) , b = sc . nextInt ( ) ;
  int M = sc . nextInt ( ) ;
  int mod = 1000000007 ;
  int [ ] node = new int [ N + 1 ] ;
  node [ a ] = 1 ;
  long [ ] count = new long [ N + 1 ] ;
  count [ a ] = 1 ;
  ArrayList < Integer > [ ] edge = new ArrayList [ N + 1 ] ;
  for ( int i = 1 ;
  i <= N ;
  i ++ ) edge [ i ] = new ArrayList < > ( ) ;
  for ( int i = 0 ;
  i < M ;
  i ++ ) {
    int x = sc . nextInt ( ) , y = sc . nextInt ( ) ;
    edge [ x ] . add ( y ) ;
    edge [ y ] . add ( x ) ;
  }
  Queue < Integer > q = new ArrayDeque < > ( ) ;
  q . add ( a ) ;
  Queue < Integer > dist = new ArrayDeque < > ( ) ;
  dist . add ( 0 ) ;
  while ( ! q . isEmpty ( ) ) {
    int vertex = q . poll ( ) ;
    int d = dist . poll ( ) ;
    for ( Integer child : edge [ vertex ] ) {
      if ( node [ child ] == 0 ) {
        q . add ( child ) ;
        dist . add ( d + 1 ) ;
        node [ child ] = d + 1 ;
      }
      if ( node [ child ] == d + 1 ) count [ child ] = ( count [ child ] + count [ vertex ] ) % mod ;
    }
  }
  out . println ( count [ b ] ) ;
}

public static void main ( String [ ] $ ) { Scanner sc = new Scanner ( in ) ; int N = sc . nextInt ( ) ; int a = sc . nextInt ( ) , b = sc . nextInt ( ) ; int M = sc . nextInt ( ) ; int mod = 1000000007 ; int [ ] node = new int [ N + 1 ] ; node [ a ] = 1 ; long [ ] count = new long [ N + 1 ] ; count [ a ] = 1 ; ArrayList < Integer > [ ] edge = new ArrayList [ N + 1 ] ; for ( int i = 1 ; i <= N ; i ++ ) edge [ i ] = new ArrayList < > ( ) ; for ( int i = 0 ; i < M ; i ++ ) { int x = sc . nextInt ( ) , y = sc . nextInt ( ) ; edge [ x ] . add ( y ) ; edge [ y ] . add ( x ) ; } Queue < Integer > q = new ArrayDeque < > ( ) ; q . add ( a ) ; Queue < Integer > dist = new ArrayDeque < > ( ) ; dist . add ( 0 ) ; while ( ! q . isEmpty ( ) ) { int vertex = q . poll ( ) ; int d = dist . poll ( ) ; for ( Integer child : edge [ vertex ] ) { if ( node [ child ] == 0 ) { q . add ( child ) ; dist . add ( d + 1 ) ; node [ child ] = d + 1 ; } if ( node [ child ] == d + 1 ) count [ child ] = ( count [ child ] + count [ vertex ] ) % mod ; } } out . println ( count [ b ] ) ; }
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