import java.util.Scanner;


public class Main {
	public static void main(String[] args) {
		Scanner scn = new Scanner(System.in);
		int N = scn.nextInt();
		int D = scn.nextInt();
		int shou = N / ( 2 * D + 1);
		int amari = N % ( 2 * D + 1);

		if (amari != 0) {
			shou++;
		}

		System.out.println(shou);
	}
}

/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz