object Main extends App {
  val sc = new java.util.Scanner(System.in)
  val n = sc.nextInt()
  val ints: Seq[Int] = Seq(-1, 0, 1).map(_ + (n / 1.08).toInt)
  println {
    ints.find(i => (i * 1.08).toInt == n).getOrElse(":(")
  }
}
