import java.io.{BufferedReader, InputStreamReader}

object Main {
  def main(args: Array[String]): Unit = {
    val in = new BufferedReader(new InputStreamReader(System.in))
    val n = in.readLine().toInt

    val min = Math.ceil(n / 1.08).toInt
    val max = Math.floor((n + 1) / 1.08).toInt

    System.out.println(
      if (min > max) {
        ":("
      } else {
        min
      }
    )
  }
}
