main = do
  n_m <- readLn :: IO Int
  loop n_m 0

loop :: Int -> Int -> IO ()
loop n_max n | xn > n_max = putStrLn ":("
             | xn == n_max = print n
             | xn < n_max = loop n_max (n+1)
  where xn = (n*108)`div`100 