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# (.NET Core 3.1.201) (judge verdict: Wrong Answer)

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

class Program
{
    #region library
    static bool debug = false;
    static int n = 0;
    static int m = 0;
    static int r = 0;
    static int c = 0;
    static int x = 0;
    static int y = 0;
    static string s = "";
    static string[] ss;
    static long sum = 0;
    static long cnt = 0;
    static long max = -1;
    static long min = 2147483647;
    static bool flg = false;
    static bool flg2 = false;
    static string alpha = "abcdefghijklmnopqrstuvwxyz";
    static long mod = 1000000007;
    static long[] arr;
    static char[,] map;
    static long[,] imap;
    public class Input
    {
        static readonly char[] separator = { ' ', '\r', '\n' };
        readonly StreamReader sr;
        readonly Queue<string> queue;

        public Input(string path = "")
        {
            queue = new Queue<string>();

            if (string.IsNullOrEmpty(path))
            {
                sr = new StreamReader(Console.OpenStandardInput());
            }
            else
            {
                sr = new StreamReader(path);
            }
        }

        public void SetText(IEnumerable<string> items)
        {
            foreach (var item in items)
            {
                SetText(item);
            }
        }

        public bool SetText(string s)
        {
            if (s == null) return false;

            foreach (var elem in s.Trim()
                .Split(separator, StringSplitOptions.RemoveEmptyEntries))
            {
                queue.Enqueue(elem);
            }

            return true;
        }

        public bool Any()
        {
            return queue.Any() || Read();
        }

        bool Read()
        {
            if (!SetText(sr.ReadLine())) return false;

            if (!queue.Any()) return Read();

            return queue.Any();
        }

        public string Next()
        {
            if (!queue.Any() && !Read())
            {
                return "";
            }

            return queue.Dequeue();
        }

        bool Accumulate(int n)
        {
            while (queue.Count() < n)
            {
                if (!Read()) return false;
            }

            return true;
        }

        public int NextInt()
        {
            return int.Parse(Next());
        }
        public long NextLong()
        {
            return long.Parse(Next());
        }
        public double NextDouble()
        {
            return double.Parse(Next());
        }

        T[] NextT<T>(int n, T offset, Func<string, T> parse, Func<T, T, T> add)
        {
            if (!Accumulate(n)) return null;

            var a = new T[n];

            for (int i = 0; i < n; i++)
                a[i] = add(parse(queue.Dequeue()), offset);

            return a;
        }

        public string[] Next(int n)
        {
            return NextT(n, "", x => x, (x, y) => x);
        }
        public int[] NextInt(int n, int offset = 0)
        {
            return NextT(n, offset, int.Parse, (x, y) => x + y);
        }
        public long[] NextLong(int n, long offset = 0)
        {
            return NextT(n, offset, long.Parse, (x, y) => x + y);
        }
        public double[] NextDouble(int n, double offset = 0.0)
        {
            return NextT(n, offset, double.Parse, (x, y) => x + y);
        }
    }
    static Input input;
    static int Int()
    {
        return input.NextInt();
    }
    static long Long()
    {
        return input.NextLong();
    }
    static double Double()
    {
        return input.NextDouble();
    }
    static string Text()
    {
        return input.Next();
    }
    static void wl(object obj = null)
    {
        Console.WriteLine(obj);
    }
    static void w(object obj)
    {
        Console.Write(obj);
    }
    static void Yes()
    {
        wl("YES");
    }
    static void No()
    {
        wl("NO");
    }
    static void Sum()
    {
        wl(sum);
    }
    static void Cnt()
    {
        wl(cnt);
    }
    static void SetArray(long num = -1)
    {
        if (n == 0)
        {
            n = Int();
        }
        arr = new long[n];
        for (int i = 0; i < n; i++)
        {
            if (num == -1)
                arr[i] = Int();
            else
                arr[i] = num;
        }
    }
    static void ShowArray()
    {
        for (int i = 0; i < arr.Length; i++)
        {
            if (i > 0) w(" ");
            w(arr[i]);
        }
        wl();
    }
    static void SetMap(char def = '@')
    {
        if (r == 0 && c == 0)
        {
            r = Int();
            c = Int();
        }
        map = new char[r, c];
        for (int i = 0; i < r; i++)
        {
            if (def == '@') s = Text();
            for (int j = 0; j < c; j++)
            {
                map[i, j] = (def == '@') ? s[j] : def;
            }
        }
    }
    static void SetIMap(long def = -1)
    {
        if (r == 0 && c == 0)
        {
            r = Int();
            c = Int();
        }
        imap = new long[r, c];
        for (int i = 0; i < r; i++)
        {
            for (int j = 0; j < c; j++)
            {
                imap[i, j] = (def == -1) ? Int() : def;
            }
        }
    }
    static void ShowMap()
    {
        for (int i = 0; i < r; i++)
        {
            for (int j = 0; j < c; j++)
            {
                w(map[i, j]);
            }
            wl();
        }
    }
    static void ShowIMap()
    {
        for (int i = 0; i < r; i++)
        {
            for (int j = 0; j < c; j++)
            {
                w(imap[i, j]);
            }
            wl();
        }
    }
    #endregion

    #region main
    static void Main(string[] args)
    {
        var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
        Console.SetOut(sw);
        input = new Input();
        try
        {
            Solve();
        }
        catch(Exception ex)
        {
            if (debug)
                wl(ex); //chcp 65001
            else
                throw ex;
        }
        Console.Out.Flush();
    }

    public static void Solve()
    {
        n = Int();
        int d = Int();

        int m = n;
        cnt = 1;
        while(true)
        {
            m = n / (int)cnt;
            if (m - 1 <= d * 2)
            {
                break;
            }
            cnt++;
        }

        Cnt();
    }

    #endregion
}