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 hani = D*2+1;
    
    int answer = N/hani;
    
    if (N%hani!=0){
      answer++;
    }
    
    System.out.println(answer);
  }
}
/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