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 d_2=d*2+1;
	if(n%d_2!=0) {
		System.out.println((n/d_2)+1);
	}else {
		System.out.println(n/d_2);
	}
}
}
/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