object Main extends scala.App {
    import java.util
    import scala.collection._
    import scala.collection.mutable
    import scala.math._

    val sc = new java.util.Scanner(System.in)
    val n = sc.nextInt()

    val (t1, t2) = ((n.toFloat * 100.0 / 108.0), ((n+1).toFloat * 100.0 / 108.0))
    val (t_min, t_max) = (t1.toInt, t2.toInt)
    val ans = (t_min to t_max).find(_.toFloat.*(1.08).toInt == n)

    ans match {
        case Some(value) => println(value)
        case None => println(":(")
    }
}