open System

[<AutoOpen>]
module Cin =
    let read f = stdin.ReadLine() |> f
    let reada f = stdin.ReadLine().Split() |> Array.map f

[<EntryPoint>]
let main _ =
    let N = read float
    let X = N / 1.08
    let d = (X * 10.0) |> int |> fun a -> a % 10
    let d2 = (X * 100.0) |> int |> fun a -> a % 10
    if d = 0 && d2 <> 0 then printfn ":("
    else Math.Round(X, 0, MidpointRounding.AwayFromZero) |> int |> printfn "%d"
    0
