fun main(args: Array<String>) {
    sumitrust2019b()
}

fun sumitrust2019b() {
    val n = readLine()!!.toLong()

    for (i in 1..n) {
        val price = Math.floor(i.toDouble() * 1.08).toLong()
        if (price == n) return println(i)
    }

    println(":(")
}
