public static void main ( String [ ] args ) {
  Scanner input = new Scanner ( System . in ) ;
  String s = input . nextLine ( ) ;
  String t = input . nextLine ( ) ;
  int count = 0 ;
  for ( int i = 0 ;
  i < s . length ( ) ;
  i ++ ) {
    if ( s . charAt ( i ) != t . charAt ( i ) ) count ++ ;
  }
  if ( count % 2 == 1 ) {
    System . out . println ( "impossible" ) ;
  }
  else {
    count = count / 2 ;
    for ( int i = 0 ;
    i < s . length ( ) ;
    i ++ ) {
      if ( s . charAt ( i ) == t . charAt ( i ) ) System . out . print ( s . charAt ( i ) ) ;
      else {
        if ( count % 2 == 0 ) System . out . print ( s . charAt ( i ) ) ;
        else System . out . print ( t . charAt ( i ) ) ;
        count -- ;
      }
    }
    System . out . println ( "" ) ;
  }
}

public static void main ( String [ ] args ) { Scanner input = new Scanner ( System . in ) ; String s = input . nextLine ( ) ; String t = input . nextLine ( ) ; int count = 0 ; for ( int i = 0 ; i < s . length ( ) ; i ++ ) { if ( s . charAt ( i ) != t . charAt ( i ) ) count ++ ; } if ( count % 2 == 1 ) { System . out . println ( " impossible " ) ; } else { count = count / 2 ; for ( int i = 0 ; i < s . length ( ) ; i ++ ) { if ( s . charAt ( i ) == t . charAt ( i ) ) System . out . print ( s . charAt ( i ) ) ; else { if ( count % 2 == 0 ) System . out . print ( s . charAt ( i ) ) ; else System . out . print ( t . charAt ( i ) ) ; count -- ; } } 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: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) condition: (binary_expression left: (identifier) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list))) update: (update_expression (identifier)) body: (block (if_statement condition: (parenthesized_expression (binary_expression left: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier))) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier))))) consequence: (expression_statement (update_expression (identifier)))))) (if_statement condition: (parenthesized_expression (binary_expression left: (binary_expression left: (identifier) right: (decimal_integer_literal)) right: (decimal_integer_literal))) consequence: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (string_literal))))) alternative: (block (expression_statement (assignment_expression left: (identifier) right: (binary_expression left: (identifier) right: (decimal_integer_literal)))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) condition: (binary_expression left: (identifier) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list))) update: (update_expression (identifier)) body: (block (if_statement condition: (parenthesized_expression (binary_expression left: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier))) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier))))) consequence: (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 (identifier)))))) alternative: (block (if_statement condition: (parenthesized_expression (binary_expression left: (binary_expression left: (identifier) right: (decimal_integer_literal)) right: (decimal_integer_literal))) consequence: (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 (identifier)))))) alternative: (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 (identifier))))))) (expression_statement (update_expression (identifier))))))) (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (string_literal))))))))