import java.util.*;
public class Main {
	Scanner sc = new Scanner(System.in);
	int a,b,c,d,e,f,k,l,m,n,x,y,z;
	int ans;
	String[] S;

	Main(){
		// 整数の入力
		n = sc.nextInt();
		d = sc.nextInt();

		// 出力
		System.out.println(n%(d*2+1)==0?n/(d*2+1):n/(d*2+1)+1);
	}

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