import java.util.Scanner

object Main {
  def main(args: Array[String]): Unit = {
    val sc = new Scanner(System.in)
    val n = sc.nextInt()

    for (i <- n * 9 / 10 to n) {
      if (math.ceil(i * 1.08) == n) {
        println(i)
        sys.exit(0)
      }
    }
    
    println(":(")
  }
}
