main = getLine >>= putStrLn . w47 . words

w47 [a,b]
 | la > lb = a
 | la < lb = b
 | p47 a b = min a b
 | otherwise = max a b
 where
  p47 x y = (\(p:_) -> p==('4','7') || p==('7','4')) $ dropWhile (\(i,j) -> i==j) (zip x y)
  la = length a
  lb = length b