public static void main ( String [ ] args ) {
  Scanner scn = new Scanner ( System . in ) ;
  String str = scn . nextLine ( ) ;
  StringBuilder sb = new StringBuilder ( ) ;
  for ( char c : str . toCharArray ( ) ) {
    if ( c != 'a' ) {
      sb . append ( c ) ;
    }
  }
  if ( sb . length ( ) % 2 == 0 ) {
    if ( sb . substring ( 0 , sb . length ( ) / 2 ) . equals ( sb . substring ( sb . length ( ) / 2 , sb . length ( ) ) ) ) {
      if ( str . substring ( str . length ( ) - sb . length ( ) / 2 , str . length ( ) ) . equals ( sb . substring ( 0 , sb . length ( ) / 2 ) ) ) {
        System . out . println ( str . substring ( 0 , str . length ( ) - sb . length ( ) / 2 ) ) ;
      }
      else {
        System . out . println ( ":(" ) ;
      }
    }
    else {
      System . out . println ( ":(" ) ;
    }
  }
  else {
    System . out . println ( ":(" ) ;
  }
}

public static void main ( String [ ] args ) { Scanner scn = new Scanner ( System . in ) ; String str = scn . nextLine ( ) ; StringBuilder sb = new StringBuilder ( ) ; for ( char c : str . toCharArray ( ) ) { if ( c != ' a ' ) { sb . append ( c ) ; } } if ( sb . length ( ) % 2 == 0 ) { if ( sb . substring ( 0 , sb . length ( ) / 2 ) . equals ( sb . substring ( sb . length ( ) / 2 , sb . length ( ) ) ) ) { if ( str . substring ( str . length ( ) - sb . length ( ) / 2 , str . length ( ) ) . equals ( sb . substring ( 0 , sb . length ( ) / 2 ) ) ) { System . out . println ( str . substring ( 0 , str . length ( ) - sb . length ( ) / 2 ) ) ; } else { System . out . println ( " : ( " ) ; } } else { System . out . println ( " : ( " ) ; } } else { System . out . println ( " : ( " ) ; } }
(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: (type_identifier) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (type_identifier) arguments: (argument_list)))) (enhanced_for_statement type: (integral_type) name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)) body: (block (if_statement condition: (parenthesized_expression (binary_expression left: (identifier) right: (character_literal))) consequence: (block (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier)))))))) (if_statement condition: (parenthesized_expression (binary_expression left: (binary_expression left: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)) right: (decimal_integer_literal)) right: (decimal_integer_literal))) consequence: (block (if_statement condition: (parenthesized_expression (method_invocation object: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (decimal_integer_literal) (binary_expression left: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)) right: (decimal_integer_literal)))) name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (binary_expression left: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)) right: (decimal_integer_literal)) (method_invocation object: (identifier) name: (identifier) arguments: (argument_list))))))) consequence: (block (if_statement condition: (parenthesized_expression (method_invocation object: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (binary_expression left: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)) right: (binary_expression left: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)) right: (decimal_integer_literal))) (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (decimal_integer_literal) (binary_expression left: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)) right: (decimal_integer_literal))))))) consequence: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (decimal_integer_literal) (binary_expression left: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)) right: (binary_expression left: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)) right: (decimal_integer_literal))))))))) alternative: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (string_literal))))))) alternative: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (string_literal))))))) alternative: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (string_literal))))))))