import java.util.*;
import java.util.Collections;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        int d = Integer.parseInt(sc.next());
        
        if(n % (2 * d + 1) == 0) {
            System.out.println((int)(n / (2 * d + 1)));
        }else {
            System.out.println((int)(n / (2 * d + 1)) + 1);
        }
        
        
    }
}
/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