Codeforces 122B - B. Lucky Substring

122B - B. Lucky Substring

calc :: String -> Int
calc s
  | n4+n7 == 0 = -1
  | otherwise  = if n4 >= n7 then 4 else 7
  where
    n4 = sum [1 | c <- s, c == '4']
    n7 = sum [1 | c <- s, c == '7']

main = do s <- getLine
          print $ calc s