public static void main ( String [ ] args ) {
  FastScanner fs = new FastScanner ( ) ;
  String s1 = fs . next ( ) ;
  String s2 = fs . next ( ) ;
  int dist = 0 ;
  for ( int i = 0 ;
  i < s1 . length ( ) ;
  i ++ ) {
    if ( s1 . charAt ( i ) != s2 . charAt ( i ) ) dist ++ ;
  }
  if ( dist % 2 != 0 ) {
    System . out . println ( "impossible" ) ;
    return ;
  }
  boolean f = true ;
  for ( int i = 0 ;
  i < s1 . length ( ) ;
  i ++ ) {
    if ( s1 . charAt ( i ) != s2 . charAt ( i ) ) {
      if ( f ) System . out . print ( s1 . charAt ( i ) ) ;
      else System . out . print ( s2 . charAt ( i ) ) ;
      f = ! f ;
    }
    else {
      System . out . print ( s1 . charAt ( i ) ) ;
    }
  }
}

public static void main ( String [ ] args ) { FastScanner fs = new FastScanner ( ) ; String s1 = fs . next ( ) ; String s2 = fs . next ( ) ; int dist = 0 ; for ( int i = 0 ; i < s1 . length ( ) ; i ++ ) { if ( s1 . charAt ( i ) != s2 . charAt ( i ) ) dist ++ ; } if ( dist % 2 != 0 ) { System . out . println ( " impossible " ) ; return ; } boolean f = true ; for ( int i = 0 ; i < s1 . length ( ) ; i ++ ) { if ( s1 . charAt ( i ) != s2 . charAt ( i ) ) { if ( f ) System . out . print ( s1 . charAt ( i ) ) ; else System . out . print ( s2 . charAt ( i ) ) ; f = ! f ; } else { System . out . print ( s1 . charAt ( i ) ) ; } } }
(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)))) (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)))) (return_statement))) (local_variable_declaration type: (boolean_type) declarator: (variable_declarator name: (identifier) value: (true))) (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: (block (if_statement condition: (parenthesized_expression (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: (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 (assignment_expression left: (identifier) right: (unary_expression operand: (identifier))))) alternative: (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 (identifier))))))))))))