let n = stdin.ReadLine() |> float

let ans =
  [ let x = n / 1.08 |> floor
    yield x
    yield x + 1. ]
  |> List.filter (fun x -> (floor (x * 1.08)) = n)
  |> (fun l ->
        match l with
        | [] -> ":("
        | x :: _ -> sprintf "%d" (int x))

ans |> stdout.WriteLine