fun main(args : Array<String>) {
    val n = readLine()!!.toDouble()
    val price = n / 1.08

    if (price == price.toInt().toDouble()) {
        println(price.toInt())
    } else {
        if (((price + 1).toInt() * 1.08).toInt() == n.toInt()) {
            println((price + 1).toInt())
            return
        }
        println(":(")
    }
}