max' :: [Integer] -> Integer
max' (a : b : _)
    | length a' > length b' = a
    | length b' > length a' = b
    | otherwise             = rmax $ zip a' b'
    where
        (a', b') = (show a, show b)
        rmax ((a'', b'') : ps)
            | a'' == '4' && b'' == '7' = a
            | a'' == '7' && b'' == '4' = b
            | a'' > b''                = a
            | b'' > a''                = b
            | otherwise                = rmax ps

main :: IO ()
main = print . max' . map read . words =<< getLine