import qualified Data.Text as T
import qualified Data.Text.IO as T
import qualified Data.Text.Read as T

main :: IO ()
main = do
  (n : _) <- map unsafeSignedDecimal . T.words <$> T.getLine :: IO [Int]
  let
    n' = toRational n :: Rational
    x' = n' * 100 / 108 :: Rational
    x0 = toRational . floor $ x' :: Rational
    x1 = x0 + 1
    n0 = floor $ x0 * 108 / 100
    n1 = floor $ x1 * 108 / 100
  putStrLn $ if n0 == n then show x0 else n1 == n then show x1 else ":("

unsafeSignedDecimal :: T.Text -> Int
unsafeSignedDecimal s = case T.signed T.decimal s of
  Right (n, _) -> n
