fun main(args: Array<String>) {
    val N = readLine()!!.toInt()
    val ans = solve(N)
    println(ans)
}

// 割り切れる場合
// 切り上げる場合
// 切り捨てる場合
fun solve(N: Int): String {
    val taxRate = 1.08
    val taxRate2 = (1.08 * 100).toInt()
    val N2 = N * 100

    val base = N / taxRate

    val floorTax = floor(base)
    val ceilTax = ceil(base)

    val floorFloor = floor(floorTax * taxRate).toInt()
    val ceilFloor = floor(ceilTax * taxRate).toInt()

    return when {
        N2 % taxRate2 == 0 -> {
            (N2 / taxRate2).toString()
        }
        floorFloor == N -> {
            floorTax.toInt().toString()
        }
        ceilFloor == N -> {
            ceilTax.toInt().toString()
        }
        else -> {
            ":("
        }
    }
}

fun floor(target: Double): Double {
    return Math.floor(target)
}

fun ceil(target: Double): Double {
    return Math.ceil(target)
}
