import java.io.*;
import java.math.*;
import java.security.*;
import java.text.*;
import java.util.*;
import java.util.concurrent.*;
import java.util.regex.*;
public class Main {
      private static final Scanner scanner = new Scanner(System.in);
    public static void main(String[] args) {
     int n = scanner.nextInt();
      int d = scanner.nextInt();
      int x=2*d+1;int y=0;
      if(n%x==0)
        y=n/x;
      else
        y=n/x+1;
      System.out.print(y);
        

    }
}

/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