public static void main ( String [ ] args ) {
  Scanner s = new Scanner ( System . in ) ;
  int rows = s . nextInt ( ) ;
  int cols = s . nextInt ( ) ;
  double d = s . nextDouble ( ) ;
  int [ ] nums = new int [ rows * cols ] ;
  for ( int i = 0 ;
  i < rows * cols ;
  i ++ ) nums [ i ] = s . nextInt ( ) ;
  Arrays . sort ( nums ) ;
  int median = nums . length % 2 == 0 ? ( nums [ nums . length / 2 - 1 ] ) : nums [ nums . length / 2 ] ;
  double moves = 0 ;
  for ( int i = 0 ;
  i < rows * cols ;
  i ++ ) {
    if ( nums [ i ] != median ) moves += Math . abs ( ( nums [ i ] - median ) / d ) ;
  }
  if ( moves == Math . ceil ( moves ) ) System . out . println ( ( int ) moves ) ;
  else System . out . println ( "-1" ) ;
}

public static void main ( String [ ] args ) { Scanner s = new Scanner ( System . in ) ; int rows = s . nextInt ( ) ; int cols = s . nextInt ( ) ; double d = s . nextDouble ( ) ; int [ ] nums = new int [ rows * cols ] ; for ( int i = 0 ; i < rows * cols ; i ++ ) nums [ i ] = s . nextInt ( ) ; Arrays . sort ( nums ) ; int median = nums . length % 2 == 0 ? ( nums [ nums . length / 2 - 1 ] ) : nums [ nums . length / 2 ] ; double moves = 0 ; for ( int i = 0 ; i < rows * cols ; i ++ ) { if ( nums [ i ] != median ) moves += Math . abs ( ( nums [ i ] - median ) / d ) ; } if ( moves == Math . ceil ( moves ) ) System . out . println ( ( int ) moves ) ; else System . out . println ( " - 1" ) ; }
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