import scala.io.StdIn

object Main {

  def main(args: Array[String]): Unit = {
    val Array(n, d) = StdIn.readLine().split(' ').map(_.toInt)
    val range = 2 * d + 1
    val remain = if (n % range == 0) 0 else 1
    val ans = n / range + remain
    println(ans)
  }

}