main = do
 a <- getLine
 let h = g.f. (read::String->Double) $ a
 --print h
 if(h /= -1) then print h else putStr ":("

f :: Double -> [Int]
f x = [if((x==).fromIntegral.floor.(1.08*).fromIntegral.ceiling $ moto) then 
	   ceiling moto else -1 ,
       if((x==).fromIntegral.floor.(1.08*).fromIntegral.floor $   moto) then 
       floor moto else -1] 
  where 
    moto = x /1.08
    
g :: [Int] -> Int
g (-1: (-1) :xs)= -1 
g (-1:a:xs) =  a
g (a:_) =  a
g _ = -3