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 sta=d*2+1;
      for(int i=0;i<20;i++){
      if(sta*i>=n){
      System.out.println(i);
        break;
      }
      }

    }
}

/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