public static void main ( String args [ ] ) {
  Scanner s = new Scanner ( System . in ) ;
  int n = s . nextInt ( ) ;
  int dp [ ] = new int [ 101 ] ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) {
    dp [ s . nextInt ( ) ] ++ ;
  }
  int a = - 1 , b = - 1 , c = - 1 ;
  for ( int i = 0 ;
  i <= 100 ;
  i ++ ) {
    if ( dp [ i ] > 0 ) {
      if ( a == - 1 ) {
        a = i ;
      }
      else if ( b == - 1 ) {
        b = i ;
      }
      else if ( c == - 1 ) {
        c = i ;
      }
      else {
        System . out . println ( - 1 ) ;
        return ;
      }
    }
  }
  if ( b == - 1 && c == - 1 ) {
    System . out . println ( 0 ) ;
    return ;
  }
  if ( c == - 1 ) {
    if ( ( b - a ) % 2 == 0 ) {
      System . out . println ( ( b - a ) / 2 ) ;
    }
    else {
      System . out . println ( b - a ) ;
    }
    return ;
  }
  if ( 2 * b == a + c ) {
    System . out . println ( b - a ) ;
  }
  else {
    System . out . println ( - 1 ) ;
  }
}

public static void main ( String args [ ] ) { Scanner s = new Scanner ( System . in ) ; int n = s . nextInt ( ) ; int dp [ ] = new int [ 101 ] ; for ( int i = 0 ; i < n ; i ++ ) { dp [ s . nextInt ( ) ] ++ ; } int a = - 1 , b = - 1 , c = - 1 ; for ( int i = 0 ; i <= 100 ; i ++ ) { if ( dp [ i ] > 0 ) { if ( a == - 1 ) { a = i ; } else if ( b == - 1 ) { b = i ; } else if ( c == - 1 ) { c = i ; } else { System . out . println ( - 1 ) ; return ; } } } if ( b == - 1 && c == - 1 ) { System . out . println ( 0 ) ; return ; } if ( c == - 1 ) { if ( ( b - a ) % 2 == 0 ) { System . out . println ( ( b - a ) / 2 ) ; } else { System . out . println ( b - a ) ; } return ; } if ( 2 * b == a + c ) { System . out . println ( b - a ) ; } else { System . out . println ( - 1 ) ; } }
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right: (identifier)))) consequence: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (binary_expression left: (identifier) right: (identifier)))))) alternative: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (unary_expression operand: (decimal_integer_literal)))))))))