import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int D = sc.nextInt();
		int ans;

		if(N%(D+1+D)>0) {
			ans = N / (D + 1 + D)+1;
		} else {
			ans = N / (D + 1 + D);
		}
		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