
import java.util.Scanner;
class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int N = Integer.parseInt(sc.next());
		int D = Integer.parseInt(sc.next());

		int a = N / (2*D + 1);
		int b = N % (2*D + 1);
		if(b == 0) {
			System.out.println(a);
		} else {
			System.out.println(a+1);
		}
	}

}

/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