import java.util.*;

public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		String point = sc.nextLine();
		
		String[] points = point.split(" ");
		int intN = Integer.parseInt(points[0]);
		int intD = Integer.parseInt(points[1]);
		
		double d2 = intD * 2.0 + 1;
		double observer = Math.ceil(intN / d2);

		System.out.println((int)observer);
	}
}
/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