import java.util.*;
public class Main{
    public static void main(String sp[]){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
		int d = sc.nextInt();
		int val = (2*d)+1;
		int div = n/val;
		int mod = n%val;
		if(mod>0)
			div+=1;
		System.out.println(div);
	}
}
/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