import           Data.Ratio
import           Data.Maybe

main :: IO ()
main = do
  n <- readLn
  case solve n of
    Nothing -> putStrLn ":("
    Just x  -> print x

solve :: Int -> Maybe Int
solve n = listToMaybe [ x | x <-[x0 .. x1], floor (x * 108 % 100) == n ]
  where
    x0 = ceiling $ n * 100 % 108
    x1 = floor $ (n + 1) * 100 % 108