class Program
{
    static void Main(string[] args)
    {
        string[] input = Console.ReadLine()!.Split(' ');
        int total = int.Parse(input[0]);
        int outNum = int.Parse(input[1]);

        if (outNum == 0 || outNum > total)
        {
            Console.WriteLine(0);
        }
        else if (total % 2 == 0)
        {
            Console.WriteLine(total - 2);
        }
        else
        {
            if (outNum == total / 2 + 1)
            {
                Console.WriteLine(total - 1);
            }
            else
            {
                Console.WriteLine(total - 2);
            }
        }
    }
}