Problem:
Score : 200 points

Problem Statement

There are N apple trees in a row. People say that one of them will bear golden apples.

We want to deploy some number of inspectors so that each of these trees will be inspected.

Each inspector will be deployed under one of the trees. For convenience, we will assign numbers from 1 through N to the trees. An inspector deployed under the i-th tree (1 \leq i \leq N) will inspect the trees with numbers between i-D and i+D (inclusive).

Find the minimum number of inspectors that we need to deploy to achieve the objective.

Constraints

All values in input are integers.

1 \leq N \leq 20

1 \leq D \leq 20

Input

Input is given from Standard Input in the following format:

N D

Output

Print the minimum number of inspectors that we need to deploy to achieve the objective.

Sample Input 1

6 2

Sample Output 1

2

We can achieve the objective by, for example, placing an inspector under Tree 3 and Tree 4.

Sample Input 2

14 3

Sample Output 2

2

Sample Input 3

20 4

Sample Output 3

3

Solution in C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Numerics;

namespace AtcoderC
{
    class Program
    {
        /*static void Main(string[] args)
        {
        }*/
        //ABC132C
        //全探索法
        /*static void Main()
        {
            BigInteger N = int.Parse(Console.ReadLine());
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int min = input.Min();
            int max = input.Max();
            int countARC = 0;
            int countABC = 0;
            int Answer = 0;
            for (int i = min; i <= max; i++)
            {
                countABC = 0;
                countARC = 0;
                foreach (int n in input)
                {
                    if (n > i)
                    {
                        countARC++;
                    }
                    else
                    {
                        countABC++;
                    }
                }
                if (countABC == countARC)
                {
                    Answer++;
                }
            }
            Console.WriteLine(Answer);
        }*/
        //2回目
        /*static void Main()
        {
            int N = int.Parse(Console.ReadLine());
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            Array.Sort(input);
            int min = input[(N / 2) - 1];
            int max = input[N / 2];
            Console.WriteLine(max - min);
        }*/

        //ABC131C
        //ユーグリッドの互除法の応用
        /*static void Main()
        {
            BigInteger[] input = Console.ReadLine().Split().Select(BigInteger.Parse).ToArray();
            BigInteger A = input[0];
            BigInteger B = input[1];
            BigInteger C = input[2];
            BigInteger D = input[3];
            BigInteger ans = (B / C) + (B / D);           
            //最小公倍数
            var ans2 = C * D / lcm(C, D);
            BigInteger ans3 = B / ans2;
            var BB = B - ans + ans3;
            ans = (A - 1) / C + (A - 1) / D;
            ans3 = (A - 1) / ans2;
            var AA = (A - 1) - ans + ans3;
            Console.WriteLine(BB - AA);
            
        }
        static public BigInteger lcm(BigInteger a, BigInteger b)
        {
            var r = a % b;
            while (r != 0)
            {
                a = b;
                b = r;
                r = a % b;
            }
            return b;
        }*/

        //129C
        //動的計画法
        //TLEになる時間がかかり過ぎる
        /*static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int N = input[0];
            int M = input[1];
            List<int> ttt = new List<int>();
            for (int i = 0;i < M;i++)
            {
                var temp = int.Parse(Console.ReadLine());
                ttt.Add(temp);
            }
            int[] dparray = new int[N+2];
            dparray[N] = 1;
            dparray[N - 1] = 1;
            for (int i = N-1; i >= 0; i--)
            {
                if (ttt.Contains(i))
                {
                    dparray[i] = 0;
                }
                dparray[i] = (dparray[i + 1] + dparray[i + 2]) % 1000000007;
            }
            Console.WriteLine(dparray[0]);
        }*/
        //ABC128
        /*static void Main()
        {

        }*/
        //ABC134A
        /*static void Main()
        {
            int r = int.Parse(Console.ReadLine());
            var ans = 3 * r * r;
            Console.WriteLine(ans);
        }*/
        //ABC134B
        static void Main()
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int N = input[0];
            int D = input[1];
            int count = 0;
            int ans = N;
            while (true)
            {
                ans = ans - (D * 2 + 1);
                count++;
                if (ans < 0)
                {
                    break;
                }
            }
            Console.WriteLine(count);
        }
    }
}