open System

[<EntryPoint>]
let main argv =
    let N = stdin.ReadLine() |> int

    let A = double N / 1.08
    let N1 = int (Math.Floor(A))
    let N2 = int (Math.Ceiling(A))

    let N3 = (N1 * 108) / 100
    let N4 = (N2 * 108) / 100

    if (N3 = N) then
        printfn "%d" N1
    else if (N4 = N) then
        printfn "%d" N2
    else
        printfn ":("
    0