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

    val calc = n / (d * 2 + 1)
    val nokori = n % (d * 2 + 1)
    if (nokori > 0) println(calc + 1)
    else println(calc)
  }
}