package net.ipipip0129.kotlin.yukicoder

import java.io.BufferedReader
import java.io.BufferedWriter
import java.io.InputStreamReader
import java.io.OutputStreamWriter

fun main(args: Array<String>) {
    val br = BufferedReader(InputStreamReader(System.`in`))
    val bw = BufferedWriter(OutputStreamWriter(System.out))

    val data = br.readLine().split(" ").map { it.toInt() }.toTypedArray()

    val len = data[0]
    val oCount = data[1]

    var count = len / oCount

    if (len % oCount == 0) count -= 1

    count /= 2
    count *= oCount

    bw.write("$count")
    bw.flush()

    br.close()
    bw.close()
}