import java.util.Scanner;

public class Main{

    public static void main(String args[]){

        Scanner scan = new Scanner(System.in);
        Integer n = Integer.parseInt(scan.next());
        Integer d = Integer.parseInt(scan.next());
 
        int x = n/(2*d+1);
        int mod = n%((2*d+1));

        if(mod == 0)
            System.out.println(n/(2*d+1));
        else
            System.out.println(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