fun main(args: Array<String>) {
    val N = nextInt()
    val X = Math.ceil(N.toDouble() / 1.08).toInt()
    val Y = Math.floor(X.toDouble() * 1.08).toInt()
    println(if (N == Y) X else ":(")
}
fun next() = readLine()!!
fun nextInt(delta: Int = 0) = Integer.parseInt(next()) + delta