import java.util.Scanner

object Main extends App {

  val sc = new Scanner(System.in)

  val n   = sc.nextInt
  val raw1 = (n / 1.08).floor.toInt
  val raw2 = (n / 1.08).ceil.toInt

  val res = Seq(raw1, raw2)
    .find{x => (x * 1.08).floor.toInt == n}
    .fold(":(")(_.toString)

  println(res)

}