public static void main ( String [ ] args ) {
  Scanner s = new Scanner ( System . in ) ;
  int n = s . nextInt ( ) ;
  int m = s . nextInt ( ) ;
  if ( m > n ) {
    System . out . println ( - 1 ) ;
    return ;
  }
  else if ( n % 2 == 0 ) {
    int k = n / 2 ;
    while ( k <= n ) {
      if ( k % m == 0 ) {
        System . out . println ( k ) ;
        return ;
      }
      k ++ ;
    }
  }
  else {
    int k = ( n / 2 ) + 1 ;
    while ( k <= n ) {
      if ( k % m == 0 ) {
        System . out . println ( k ) ;
        return ;
      }
      k ++ ;
    }
  }
}

public static void main ( String [ ] args ) { Scanner s = new Scanner ( System . in ) ; int n = s . nextInt ( ) ; int m = s . nextInt ( ) ; if ( m > n ) { System . out . println ( - 1 ) ; return ; } else if ( n % 2 == 0 ) { int k = n / 2 ; while ( k <= n ) { if ( k % m == 0 ) { System . out . println ( k ) ; return ; } k ++ ; } } else { int k = ( n / 2 ) + 1 ; while ( k <= n ) { if ( k % m == 0 ) { System . out . println ( k ) ; return ; } k ++ ; } } }
(method_declaration (modifiers) type: (void_type) name: (identifier) parameters: (formal_parameters (formal_parameter type: (array_type element: (type_identifier) dimensions: (dimensions)) name: (identifier))) body: (block (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (type_identifier) arguments: (argument_list (field_access object: (identifier) field: (identifier)))))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (if_statement condition: (parenthesized_expression (binary_expression left: (identifier) right: (identifier))) consequence: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (unary_expression operand: (decimal_integer_literal))))) (return_statement)) alternative: (if_statement condition: (parenthesized_expression (binary_expression left: (binary_expression left: (identifier) right: (decimal_integer_literal)) right: (decimal_integer_literal))) consequence: (block (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (binary_expression left: (identifier) right: (decimal_integer_literal)))) (while_statement condition: (parenthesized_expression (binary_expression left: (identifier) right: (identifier))) body: (block (if_statement condition: (parenthesized_expression (binary_expression left: (binary_expression left: (identifier) right: (identifier)) right: (decimal_integer_literal))) consequence: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (identifier)))) (return_statement))) (expression_statement (update_expression (identifier)))))) alternative: (block (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (binary_expression left: (parenthesized_expression (binary_expression left: (identifier) right: (decimal_integer_literal))) right: (decimal_integer_literal)))) (while_statement condition: (parenthesized_expression (binary_expression left: (identifier) right: (identifier))) body: (block (if_statement condition: (parenthesized_expression (binary_expression left: (binary_expression left: (identifier) right: (identifier)) right: (decimal_integer_literal))) consequence: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (identifier)))) (return_statement))) (expression_statement (update_expression (identifier))))))))))