fun main(args: Array<String>) {
    val (N) = readLine()!!.split(' ').map(String::toLong)
    val t = Math.floor(N / 1.08).toLong()
    for (i in -3..3) {
        if (Math.floor((t + i) * 1.08).toLong() == N) {
            println(t+i)
            return
        }
    }
    println(":(")
}