main::IO()
main = do
  n:m:[]<-fmap words getLine
  let cComp ('4','7') = GT
      cComp ('7','4') = LT
      cComp (a,b) = a `compare` b
      f1 (x:xs)
       |x ==LT    = m
       |x ==GT    = n
       |otherwise = f1 xs
      exe n m
        |cmp==GT    = n
        |cmp==LT    = m
        |otherwise  = f1 . map cComp $zip n m
        where cmp =(length n) `compare` (length m)

  putStrLn $ exe n m