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.Linq;
using System.Text.RegularExpressions;
using System.Collections.Generic;
using static program.Input;
using static program.MyMath;
using static System.Console;
using static program.Const;

namespace program
{
  class Program
  {
    static void Main()
    {
      var n = ScanIntArray();
      WriteLine(Math.Max(0, n[0] - 2*n[1]));
    }
  }

  static class Const
  {
    public static string alp = "abcdefghijklmnopqrstuvwxyz";
    public static string Alp = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    public static long MOD = 1000000007;
    public static int INF = int.MaxValue;
    public static long INFL = long.MaxValue;
  }
  static class Input
  {
    static public int ScanInt() { return int.Parse(Console.ReadLine()); }
    static public uint ScanUint() { return uint.Parse(Console.ReadLine()); }
    static public double ScanDouble() { return double.Parse(Console.ReadLine()); }
    static public long ScanLong() { return long.Parse(Console.ReadLine()); }
    static public ulong ScanUlong() { return ulong.Parse(Console.ReadLine()); }
    static public int[] ScanIntArray() { return Console.ReadLine().Split().Select(x => int.Parse(x)).ToArray(); }
    static public uint[] ScanUintArray() { return Console.ReadLine().Split().Select(x => uint.Parse(x)).ToArray(); }
    static public double[] ScanDoubleArray() { return Console.ReadLine().Split().Select(x => double.Parse(x)).ToArray(); }
    static public long[] ScanLongArray() { return Console.ReadLine().Split().Select(x => long.Parse(x)).ToArray(); }
    static public ulong[] ScanUlongArray() { return Console.ReadLine().Split().Select(x => ulong.Parse(x)).ToArray(); }

    static public string ScanStr() { return Console.ReadLine(); }
    static public string[] ScanStrArray() { return ScanStr().Split(); }
    static public string RegaxReplace(this string arg1, string pattern, string replacement) { return Regex.Replace(arg1, pattern, replacement); }
    static public string Join<T>(this IEnumerable<T> source, string separator) { return string.Join(separator, source); }
  }

  static class MyMath
  {
    static public int Floor2(this int a)
    {
      int N = 1;
      while (N < a) N *= 2;
      return N / 2;
    }
    static public long Floor2(this long a)
    {
      long N = 1;
      while (N < a) N *= 2;
      return N / 2;
    }
    static public int Ceiling2(this int a)
    {
      int N = 1;
      while (N < a) N *= 2;
      return N;
    }
    static public long Ceiling2(this long a)
    {
      long N = 1;
      while (N < a) N *= 2;
      return N;
    }
    static public long[] CumSum(this long[] a)
    {
      var res = new long[a.Length];
      long s = 0;
      for (int i = 0; i < a.Length; i++) res[i] = (s += a[i]);
      return res;
    }
    static public IEnumerable<int> PrimeFactors(this int n)
    {
      int tmp = n;
      for (int i = 2; i * i <= n; i++)
      {
        while (tmp % i == 0)
        {
          tmp /= i;
          yield return i;
        }
      }
      if (tmp != 1) yield return tmp;
    }
    static public IEnumerable<long> PrimeFactors(this long n)
    {
      var tmp = n;
      for (long i = 2; i * i <= n; i++)
      {
        while (tmp % i == 0)
        {
          tmp /= i;
          yield return i;
        }
      }
      if (tmp != 1) yield return tmp;
    }
    static public int Digit(this int n, int d) { if (d < 0) return 0; return (n % (int)Math.Pow(10, d + 1)) / (int)Math.Pow(10, d); }
    static public int Length(this int n) { if (n < 0) return 0; return (int)Math.Log10(n) + 1; }
    static public int Top(this int n) { return n.Digit(n.Length() - 1); }
    public static bool isPrime(this int p)
    {
      if (p <= 1) return false;
      for (int i = 2; i * i <= p; i++)
        if (p % i == 0) return false;
      return true;
    }
    public static bool isPrime(this long p)
    {
      if (p <= 1) return false;
      for (long i = 2; i * i <= p; i++)
        if (p % i == 0) return false;
      return true;
    }
    public static int GCD(int a, int b)
    {
      Func<int, int, int> gcd = null;
      gcd = (x, y) => y == 0 ? x : gcd(y, x % y);
      return a > b ? gcd(a, b) : gcd(b, a);
    }
    public static long GCD(long a, long b)
    {
      Func<long, long, long> gcd = null;
      gcd = (x, y) => y == 0 ? x : gcd(y, x % y);
      return a > b ? gcd(a, b) : gcd(b, a);
    }
    public static ulong extGCD(ulong a, ulong b, ref ulong x, ref ulong y)
    {
      if (b == 0)
      {
        x = 1;
        y = 0;
        return a;
      }
      var d = extGCD(b, a % b, ref y, ref x);
      y -= a / b * x;
      return d;
    }
    public static int LCM(int a, int b) { return a * b / GCD(a, b); }
    public static long LCM(long a, long b) { return a * b / GCD(a, b); }
    public static long Inv(this long a, long m = 1000000007)
    {
      long b = m, u = 1, v = 0;
      while (b != 0)
      {
        long t = a / b;
        a -= t * b;
        swap(ref a, ref b);
        u -= t * v;
        swap(ref u, ref v);
      }
      u %= m;
      if (u < 0) u += m;
      return u;
    }
    private static void swap(ref long a, ref long b)
    {
      var t = a;
      a = b;
      b = t;
    }
  }


}