import java.util.*;
public class Main {
    public static void main(String[] args) {
    
    Scanner sc = new Scanner(System.in);
      
      //入力
      int N = sc.nextInt();//木の本数
      int D = sc.nextInt();
      
      //1人の監視員が監視できる範囲
      int one = D*2+1;
      
      //監視員が最低何人必要か求める
      int min = N/(D*2+1);
      if(N%one!=0){
        	System.out.println(min+1);
      }
      else{
        	System.out.println(min);
    }
    }
}
/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