import Data.Bool

main :: IO ()
main = do
  { [n] <- readLineInputs
  ; putStrLn $ solve n
  }

{- |
>>> solve 432
"400"
>>> solve 1079
":("
>>> solve 1001
"927"
-}
solve :: Int -> String
solve n = bool ":(" (show n') (n == floor (fromIntegral n' * 1.08))
  where
    n' = ceiling $ fromIntegral n / 1.08

readLineInputs :: Read a => IO [a]
readLineInputs = map read . words <$> getLine
