import java.util.Scanner;

public class Main {
	Scanner scan = new Scanner(System.in);
	
	
	void compute() {
		int N = scan.nextInt();
		int D = scan.nextInt();
		double x = (double)N / (double)(2 * D + 1);
		int y = N / (2 * D + 1);
		if(0 < x - y) {
			y++;
		}
		System.out.println(y);
	}

	public static void main(String[] args) {
		new Main().compute();
	}

}

/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