import java.util.Scanner;

public class Main {
	public static void main(String[] args){
		Scanner stdIn = new Scanner(System.in);
		int N = stdIn.nextInt();
		int D= stdIn.nextInt();
		int ans = 0;
		int cnt = 0;
		cnt = D * 2 + 1;
		ans = N / cnt;
		if(N % cnt == 0){
			System.out.println(ans);
		}
		else{
			System.out.println(ans + 1);
		}
		stdIn.close();
	}
}
/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