import java.util.Scanner;
 
public class Main { //クラス名はMain
    public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    String n = scan.next();
    String d = scan.next();
    int N = Integer.parseInt(n);
    int D = Integer.parseInt(d);
    
    int ans = (N + 2*D) / (2*D +1);
    
    System.out.println(ans);
 
 
    }
}
/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