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

class Program
{
    private static readonly Scanner sc = new Scanner();
    static void Main(string[] args)
    {
        int n = sc.Int;
        int k = sc.Int;

        if (k == 0 || k > n)
        {
            Console.WriteLine(0);
        }
        else
        {
            Console.WriteLine(n % 2 == 1 && n / 2 + 1 == k ? n-1 : n-2);
        }
    }
}

class Scanner
{
    private string[] _str = new string[0];
    private int _i;

    public string Line { get { return Console.ReadLine(); } }

    public string Str
    {
        get
        {
            if (_i >= _str.Length)
            {
                _str = Line.Split(' ');
                _i = 0;
            }
            return _str[_i++];
        }
    }

    public string[] StrArr { get { return Line.Split(' '); } }
    public int Int { get { return int.Parse(Str); } }
    public int[] IntArr { get { return Line.Split(' ').Select(int.Parse).ToArray(); } }
    public long Long { get { return long.Parse(Str); } }
    public long[] LongArr { get { return Line.Split(' ').Select(long.Parse).ToArray(); } }
    public double Double { get { return double.Parse(Str); } }
    public double[] DoubleArr { get { return Line.Split(' ').Select(double.Parse).ToArray(); } }
}