fun main(arr:Array<String>) {
    val n = readLine()!!.toInt()
    val base = (n / 1.08).toInt()
    val ans = listOf(base, base+1).filter { (it*1.08).toInt() == n }
    if(ans.isEmpty()) {
        println(":(")
    } else {
        println(ans.first())
    }
}