fun main(args: Array<String>) {
    p134()
}

fun p134() {
    val (l, k) = readLine()!!.split(' ').map { it.toLong() }

    var remain = l
    var answer = 0L

    while (remain > 2 * k) {
        answer += k
        remain -= 2 * k
    }

    println(answer)
}