Problem:
Score : 100 points

Problem Statement

The window of Takahashi's room has a width of A. There are two curtains hung over the window, each of which has a horizontal length of B. (Vertically, the curtains are long enough to cover the whole window.)

We will close the window so as to minimize the total horizontal length of the uncovered part of the window.
Find the total horizontal length of the uncovered parts of the window then.

Constraints

1 \leq A \leq 100

1 \leq B \leq 100

A and B are integers.

Input

Input is given from Standard Input in the following format:

A B

Output

Print the total horizontal length of the uncovered parts of the window.

Sample Input 1

12 4

Sample Output 1

4

We have a window with a horizontal length of 12, and two curtains, each of length 4, that cover both ends of the window, for example. The uncovered part has a horizontal length of 4.

Sample Input 2

20 15

Sample Output 2

0

If the window is completely covered, print 0.

Sample Input 3

20 30

Sample Output 3

0

Each curtain may be longer than the window.

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;
using static System.Console;
using static System.Math;

namespace Atcoder
{
    class Program
    {
        static void Main(string[] args)
        {
            try
            {
                Solve.Answer();
            }
            catch (Exception ex)
            {
                WriteLine(ex.ToString());
            }
        }
    }

    public class Solve
    {
        public static void Answer()
        {
            var ab = IO.ReadSplitInt;
            WriteLine(ab[0] - ab[1] * 2 >= 0 ? ab[0] - ab[1] * 2 : 0);
        }

        public class IO
        {
            #region Input
            public static string ReadString => ReadLine();
            public static int ReadInt => int.Parse(ReadLine());
            public static long ReadLong => long.Parse(ReadLine());
            public static double ReadDouble => double.Parse(ReadLine());

            public static List<string> ReadSplitString => ReadLine().Split().ToList();
            public static List<int> ReadSplitInt => ReadLine().Split().Select(s => int.Parse(s)).ToList();
            public static List<long> ReadSplitLong => ReadLine().Split().Select(s => long.Parse(s)).ToList();
            public static List<double> ReadSplitdouble => ReadLine().Split().Select(s => double.Parse(s)).ToList();
            #endregion

            #region Output
            public static void OutJoinSpace<T>(IEnumerable<T> list) => WriteLine(string.Join(" ", list));
            public static void WriteLineYes_or_No(bool flag) => WriteLine(flag ? "Yes" : "No");

            #endregion
        }

        /// <summary>
        /// 正の数の四則演算のみ対応
        /// </summary>
        public class Mod
        {
            // 割る数の定義
            private readonly static long M = 1000000007;

            // 足し算
            public static long Add(long a, long b) => (a * b) % M;
            // 引き算
            public static long Sub(long a, long b) => (a - b) % M >= 0 ? (a - b) % M : (a - b) % M + M;
            // 掛け算
            public static long Multi(long a, long b) => (a * b) % M;
        }

        public class Scanner
        {
            public string Str => Console.ReadLine().Trim();
            public int Int => int.Parse(Str);
            public long Long => long.Parse(Str);
            public double Double => double.Parse(Str);
            public int[] ArrInt => Str.Split(' ').Select(int.Parse).ToArray();
            public long[] ArrLong => Str.Split(' ').Select(long.Parse).ToArray();
            //public char[][] Grid(int n) => Create(n, () => Str.ToCharArray());
            //public int[] ArrInt1D(int n) => Create(n, () => Int);
            //public long[] ArrLong1D(int n) => Create(n, () => Long);
            //public int[][] ArrInt2D(int n) => Create(n, () => ArrInt);
            //public long[][] ArrLong2D(int n) => Create(n, () => ArrLong);
            private Queue<string> q = new Queue<string>();
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            // インライン化を推奨する
            public T Next<T>()
            {
                if (q.Count == 0) foreach (var item in Str.Split(' ')) q.Enqueue(item);
                return (T)Convert.ChangeType(q.Dequeue(), typeof(T));
            }
            public void Make<T1>(out T1 v1) => v1 = Next<T1>();
            public void Make<T1, T2>(out T1 v1, out T2 v2) { v1 = Next<T1>(); v2 = Next<T2>(); }
            public void Make<T1, T2, T3>(out T1 v1, out T2 v2, out T3 v3) { Make(out v1, out v2); v3 = Next<T3>(); }
            public void Make<T1, T2, T3, T4>(out T1 v1, out T2 v2, out T3 v3, out T4 v4) { Make(out v1, out v2, out v3); v4 = Next<T4>(); }
            public void Make<T1, T2, T3, T4, T5>(out T1 v1, out T2 v2, out T3 v3, out T4 v4, out T5 v5) { Make(out v1, out v2, out v3, out v4); v5 = Next<T5>(); }
            public void Make<T1, T2, T3, T4, T5, T6>(out T1 v1, out T2 v2, out T3 v3, out T4 v4, out T5 v5, out T6 v6) { Make(out v1, out v2, out v3, out v4, out v5); v6 = Next<T6>(); }
        }
    }

    public class Combination
    {
        public static long Choice2(long n)
        {
            if (n <= 1)
                return 0;
            if (n == 2)
                return 1;
            else
                return n * (n - 1) / 2;
        }
    }
}