import java.util.Scanner

fun main(args: Array<String>) {
    val sc = Scanner(System.`in`)

    val allTreeNum = sc.nextInt()
    val d = sc.nextInt()
  	
  	val rangePerPerson = 2*d + 1
  	
  	val i = if((allTreeNum % rangePerPerson) > 0) {
      1
    } else { 0 }
  
  	print((allTreeNum / rangePerPerson) + i)
  
}