import scala.math.{ceil, floor}

object Main extends App {

  val sc = new java.util.Scanner(System.in)
  val N = sc.nextInt()
  val X = ceil(N / 1.08 ).toInt
  if (floor(X * 1.08) == N) {
    print(X)
  } else {
    print(":(")
  }
}
