import scala.math.ceil

object Main {
  val sc = new java.util.Scanner(System.in)

  def sin(n: Int): Seq[Int] = Seq.fill(n)(sc.nextInt())

  def iin(): Int = sc.nextLine().toInt

  def main(args: Array[String]): Unit = {
    val n, d = sc.nextDouble()

    println( ceil(n / (d * 2 + 1)).toInt )
  }
}