object Main {
  def main(args: Array[String]) {
    val sc = new java.util.Scanner(System.in)
    val L, K = sc.nextInt

/*
    val d = L / (K * 2)
    val m = L % (K * 2)
    if (m == 0) {
      println(K * (d-1))
    }
    else {
      println(K * d)
    }
 */
    val ans = ((L - 1) / (2 * K)) * K
    println(ans)
  }
}