import java.util.*
fun main(args: Array<String>){
	val cin = Scanner(System.`in`)
    val n = cin.nextInt()
    val d = cin.nextInt()
    val k = 2*d + 1

    if (n%k == 0) {
        println(n/k)
    } else {
        println(n/k + 1)
    }
}