import java.util.*;

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

        int N = sc.nextInt();
        int D = sc.nextInt();

        println((int)Math.ceil((double)N/(D*2+1)));
    }

    static void println(Object o){
        System.out.println(o);
    }
    static void print(Object o){
        System.out.print(o);
    }
}

/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