public static void main ( String [ ] args ) throws IOException {
  BufferedReader br = new BufferedReader ( new InputStreamReader ( System . in ) ) ;
  String [ ] temp = br . readLine ( ) . split ( " " ) ;
  int rows = Integer . parseInt ( temp [ 0 ] ) ;
  int columns = Integer . parseInt ( temp [ 1 ] ) ;
  int diff = Integer . parseInt ( temp [ 2 ] ) ;
  int [ ] numbers = new int [ rows * columns ] ;
  for ( int i = 0 ;
  i < rows ;
  i ++ ) {
    String [ ] temp2 = br . readLine ( ) . split ( " " ) ;
    for ( int j = 0 ;
    j < columns ;
    j ++ ) {
      numbers [ i * columns + j ] = Integer . parseInt ( temp2 [ j ] ) ;
    }
  }
  boolean possible = true ;
  int mod = numbers [ 0 ] % diff ;
  for ( int i = 0 ;
  i < numbers . length ;
  i ++ ) {
    if ( numbers [ i ] % diff != mod ) {
      possible = false ;
      break ;
    }
  }
  if ( possible ) {
    Arrays . sort ( numbers ) ;
    int median = numbers [ numbers . length / 2 ] ;
    int moves = 0 ;
    for ( int i = 0 ;
    i < numbers . length ;
    i ++ ) {
      moves += ( Math . abs ( median - numbers [ i ] ) ) / diff ;
    }
    System . out . println ( moves ) ;
  }
  else {
    System . out . println ( "-1" ) ;
  }
}

public static void main ( String [ ] args ) throws IOException { BufferedReader br = new BufferedReader ( new InputStreamReader ( System . in ) ) ; String [ ] temp = br . readLine ( ) . split ( " ▁ " ) ; int rows = Integer . parseInt ( temp [ 0 ] ) ; int columns = Integer . parseInt ( temp [ 1 ] ) ; int diff = Integer . parseInt ( temp [ 2 ] ) ; int [ ] numbers = new int [ rows * columns ] ; for ( int i = 0 ; i < rows ; i ++ ) { String [ ] temp2 = br . readLine ( ) . split ( " ▁ " ) ; for ( int j = 0 ; j < columns ; j ++ ) { numbers [ i * columns + j ] = Integer . parseInt ( temp2 [ j ] ) ; } } boolean possible = true ; int mod = numbers [ 0 ] % diff ; for ( int i = 0 ; i < numbers . length ; i ++ ) { if ( numbers [ i ] % diff != mod ) { possible = false ; break ; } } if ( possible ) { Arrays . sort ( numbers ) ; int median = numbers [ numbers . length / 2 ] ; int moves = 0 ; for ( int i = 0 ; i < numbers . length ; i ++ ) { moves += ( Math . abs ( median - numbers [ i ] ) ) / diff ; } System . out . println ( moves ) ; } else { System . out . println ( " - 1" ) ; } }
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