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

public class Program
{

	public void Proc()
	{
		long[] inpt = Reader.ReadLine().Split(' ').Select(a => long.Parse(a)).ToArray();

		int boxCount = 0;

		long ans = 0;
		if (inpt[1] == 0 || inpt[1] > inpt[0])
		{
            ans = 0;
		}
		else if (inpt[0] % 2 == 1 && inpt[1] == (inpt[0] + 1) / 2)
		{
			ans = inpt[0] - 1;
		}
		else
		{
			ans = Math.Max(inpt[0] - 2, 0);
		}

		Console.WriteLine((ans));
	}



	public class Reader
	{
		private static StringReader sr;
		public static bool IsDebug = false;
		public static string ReadLine()
		{
			if (IsDebug)
			{
				if (sr == null)
				{
					sr = new StringReader(InputText.Trim());
				}
				return sr.ReadLine();
			}
			else
			{
				return Console.ReadLine();
			}
		}
		private static string InputText = @"

5 2



";
	}

	public static void Main(string[] args)
	{
#if DEBUG
		Reader.IsDebug = true;
#endif
		Program prg = new Program();
		prg.Proc();
	}
}