import java.util.*

fun main(args: Array<String>) {
  val (n, d) = readLine()!!.split(" ").map(String::toInt)
  val r = d * 2 + 1
  var ans = n / r
  if (n % r > 0)
    ++ans
  println(ans)
}

fun debug(vararg objs: Any) {
  System.err.println(Arrays.deepToString((objs)))
}