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

public class yukicoder
{
    public static void Main()
    {
        double[] foo = Console.ReadLine().Split(' ').Select(x => double.Parse(x)).ToArray();
        double a = foo[0];
        double b = foo[1];

        if(b == 0 || b > a)
        {
            Console.WriteLine(0);
        }
        else if(a % 2 != 0 && b == Math.Ceiling(a / 2))
        {
            Console.WriteLine(a - 1);
        }
        else if(b >= 1 && b <= a)
        {
            Console.WriteLine(a - 2);
        }
    }
}