import java.util.*;

public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        String point = sc.nextLine();

        String[] points = point.split(" ");
        int n = Integer.parseInt(points[0]);
        int d = Integer.parseInt(points[1]);

        int d2 = d * 2 + 1;

        double d3 = (double)n / d2;

        //きりあげ
        double kiriage = Math.ceil(d3);

        System.out.println((int)kiriage);
    }
}
/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