let read f = stdin.ReadLine() |> f
let n = read float
let f a = a * 1.08 |> int
match n / 1.08 |> int with
| x' when f (float x') = int n -> string x'
| x' when f (float (x'-1)) = int n -> string (x'-1)
| x' when f (float (x'+1)) = int n -> string (x'+1)
| _ -> ":("
|> printfn "%s"
