import java.util.Scanner;

public class Main{

	public static void main(String[] args) {

		Scanner scanner = new Scanner(System.in);

		String[] tmp = scanner.nextLine().split(" ");
		int n = Integer.parseInt(tmp[0]);
		int d = Integer.parseInt(tmp[1]);

		double range = d * 2 + 1;
		double ra = n / range;
		int ans = (int)Math.ceil(ra);
		System.out.println(ans);


	}
}

/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