[<AutoOpen>]
module Program

open System

let read f = stdin.ReadLine() |> f
let reada f = stdin.ReadLine().Split() |> Array.map f
let reads f = stdin.ReadLine().Split() |> Array.toList |> List.map f

[<EntryPoint>]
let main _ =
    let N = read int

    let xs =
        [1 .. 50000]
        |> List.filter (fun x -> (float x * 1.08 |> int) = N)

    if List.isEmpty xs then
        printfn ":("
    else
        printfn "%i" (List.head xs)
    0