import java.util.*

fun main(args: Array<String>) {
  val sc = Scanner(System.`in`)
  val n = sc.nextInt()

  println(b(n))
}

private fun b(n: Int): String {
  var ans = -1
  for (i in 1..50000) {
    if (i * 108 / 100 == n) {
      ans = i
      break;
    }
  }
  if (ans == -1) return ":("

  return ans.toString()
}
