public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  int H = sc . nextInt ( ) ;
  int W = sc . nextInt ( ) ;
  int h = sc . nextInt ( ) ;
  int w = sc . nextInt ( ) ;
  int [ ] [ ] matrix = new int [ H ] [ W ] ;
  for ( int i = 0 ;
  i < H ;
  i ++ ) for ( int j = 0 ;
  j < W ;
  j ++ ) matrix [ i ] [ j ] = 0 ;
  for ( int i = 0 ;
  i < h ;
  i ++ ) {
    for ( int j = 0 ;
    j < W ;
    j ++ ) matrix [ i ] [ j ] = 1 ;
  }
  for ( int i = 0 ;
  i < H ;
  i ++ ) {
    for ( int j = 0 ;
    j < w ;
    j ++ ) matrix [ i ] [ j ] = 1 ;
  }
  int count = 0 ;
  for ( int i = 0 ;
  i < H ;
  i ++ ) {
    for ( int j = 0 ;
    j < W ;
    j ++ ) {
      if ( matrix [ i ] [ j ] == 0 ) count ++ ;
    }
  }
  System . out . println ( count ) ;
  sc . close ( ) ;
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; int H = sc . nextInt ( ) ; int W = sc . nextInt ( ) ; int h = sc . nextInt ( ) ; int w = sc . nextInt ( ) ; int [ ] [ ] matrix = new int [ H ] [ W ] ; for ( int i = 0 ; i < H ; i ++ ) for ( int j = 0 ; j < W ; j ++ ) matrix [ i ] [ j ] = 0 ; for ( int i = 0 ; i < h ; i ++ ) { for ( int j = 0 ; j < W ; j ++ ) matrix [ i ] [ j ] = 1 ; } for ( int i = 0 ; i < H ; i ++ ) { for ( int j = 0 ; j < w ; j ++ ) matrix [ i ] [ j ] = 1 ; } int count = 0 ; for ( int i = 0 ; i < H ; i ++ ) { for ( int j = 0 ; j < W ; j ++ ) { if ( matrix [ i ] [ j ] == 0 ) count ++ ; } } System . out . println ( count ) ; sc . close ( ) ; }
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