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: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;

namespace debug
{
    public static partial class Program
    {
        public static void Main()
        {
            var nd = Console.ReadLine().ToLongArray();
            var n = nd[0];
            var d = nd[1];

            Console.WriteLine(Math.Ceiling(n / (d * 2.0 + 1)));
        }
    }

    public static partial class Program
    {
        //テンプレここから
        private static long GCD(long aaa, long bbb) => bbb == 0 ? aaa : GCD(bbb, aaa % bbb);
        public static long LCM(long aaa, long bbb) => aaa * bbb / GCD(aaa, bbb);
        public static long ToLong(this double ddd) => (long) ddd;
        public static long ToLong(this string str) => long.Parse(str);
        public static long[] ToLong(this IEnumerable<string> strs) => strs.Select(x => x.ToLong()).ToArray();
        public static long[] ToLongArray(this string str, char opr = ' ') => str.Split(opr).ToLong();
        public static long Diff(long aaa, long bbb) => Math.Abs(Math.Max(aaa, bbb) - Math.Min(aaa, bbb));
        public static long Sign(this long iii) => Math.Sign(iii);
        public static bool IsEven(this long iii) => iii % 2 == 0;
        public static readonly int[] Dx = {0, -1, 0, 1};
        public static readonly int[] Dy = {-1, 0, 1, 0};
        public const int MOD = 1000000007;
        public const string ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        public const string alphabet = "abcdefghijklmnopqrstuvwxyz";
        public static long ToLong(this char ccc) => ccc.ToString().ToLong();
        public static string Join(this IEnumerable<string> array, string sep = "") => string.Join(sep, array);
        public static string Join(this IEnumerable<char> array, string sep = "") => string.Join(sep, array);

        public static string JoinToStr(this IEnumerable<long> array, string sep = "")
            => array.Select(xxx => xxx.ToString()).Join(sep);

        //テンプレここまで
    }
}