let n = stdin.ReadLine() |> int 

let mightAns =
    n
    |> float
    |> fun x -> x / 1.08 
    |> ceil
    |> int

mightAns
|> float
|> (*) 1.08
|> int
|> function
| x when x = n -> string mightAns
| _ -> ":( "
|> stdout.WriteLine