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);
        }
    }
}


Measured memory_bytes: 11220