fun main(args: Array<String>) {
    val a = readLine()!!.toInt()

    val b = (a.toDouble() / 1.08).toInt()
    if((b * 1.08).toInt() == a){
        println(b.toInt())
    }else if(((b + 1) * 1.08).toInt() == a){
        println((b+1).toInt())
    }else{
        println(":(")
    }
}
