import kotlin.math.ceil
import kotlin.math.floor

fun main(args: Array<String>) {
    //val n = 1001
    val n = readLine()!!.toInt()

    val tax = 1.08

    val expect1 = ceil(n / tax).toInt()
    val expect2 = floor(n / tax).toInt()
    
    when (n) {
        floor(expect1 * tax).toInt() -> {
            println("$expect1")
        }
        floor(expect2 * tax).toInt() -> {
            println("$expect2")
        }
        else -> println(":(")
    }
}
