-- yukicoder My Practice
-- author: Leonardone @ NEEDSDKASU
-- スマホから投稿テスト

import Data.List

main = do
   ab <- getLine
   putStrLn $ solve ab

solve ab =
   let
      (a:b:_) = words ab
      f (x:xs) (y:ys) = case (x, y) of
         ('7', '4') -> False
         ('4', '7') -> True
         _            -> case compare x y of
            LT -> False
            EQ -> f xs ys
            GT -> True
      f _ _ = True
      la = length a
      lb = length b
      r = case compare la lb of
         LT -> False
         EQ -> f a b
         GT -> True
   in
      if r then a else b