public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  int i , j ;
  double p , q , n , a , b , c , d , ans ;
  int pX [ ] = new int [ 12 ] ;
  int pY [ ] = new int [ 12 ] ;
  ans = 10000000 ;
  p = sc . nextInt ( ) ;
  q = sc . nextInt ( ) ;
  n = sc . nextInt ( ) ;
  for ( i = 0 ;
  i < n ;
  i ++ ) {
    pX [ i ] = sc . nextInt ( ) ;
    pY [ i ] = sc . nextInt ( ) ;
  }
  for ( i = 0 ;
  i < n ;
  i ++ ) {
    j = ( i < n - 1 ) ? i + 1 : 0 ;
    a = pX [ j ] - pX [ i ] ;
    b = pY [ j ] - pY [ i ] ;
    c = Math . sqrt ( a * a + b * b ) ;
    d = ( a * ( q - pY [ i ] ) + b * ( pX [ i ] - p ) ) / c ;
    d = ( d >= 0 ) ? d : - d ;
    if ( ans > d ) ans = d ;
  }
  System . out . println ( ans ) ;
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; int i , j ; double p , q , n , a , b , c , d , ans ; int pX [ ] = new int [ 12 ] ; int pY [ ] = new int [ 12 ] ; ans = 10000000 ; p = sc . nextInt ( ) ; q = sc . nextInt ( ) ; n = sc . nextInt ( ) ; for ( i = 0 ; i < n ; i ++ ) { pX [ i ] = sc . nextInt ( ) ; pY [ i ] = sc . nextInt ( ) ; } for ( i = 0 ; i < n ; i ++ ) { j = ( i < n - 1 ) ? i + 1 : 0 ; a = pX [ j ] - pX [ i ] ; b = pY [ j ] - pY [ i ] ; c = Math . sqrt ( a * a + b * b ) ; d = ( a * ( q - pY [ i ] ) + b * ( pX [ i ] - p ) ) / c ; d = ( d >= 0 ) ? d : - d ; if ( ans > d ) ans = d ; } System . out . println ( ans ) ; }
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