
fun main(args: Array<String>) {
    val n1 = readLine()!!.toInt()
    solve(n1)
}

fun solve(n1: Int){
    // (ans / 1.08).floor() = n1
    val moto = (n1 / 1.08).toInt()

    val kouho = Math.floor(moto * 1.08)
    val kouho2 = Math.floor((moto + 1) * 1.08)

    var ans = ""
    if( kouho.toInt() == n1 ) {
        ans = moto.toString()
    } else if(kouho2.toInt() == n1) {
        ans = (moto + 1).toString()
    }else{
        ans = ":("
    }
    println(ans)
}