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

class Program
{
    public void Solve()
    {
        int[] NK = Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();

        if (NK[1] == 0 || NK[0] < NK[1])
        {
            Console.WriteLine(0);
            return;
        }

        if (NK[0] % 2 == 1 && NK[0] / 2 + 1 == NK[1])
        {
            Console.WriteLine(NK[0] - 1);
            return;
        }

        Console.WriteLine(NK[0] - 2);
    }

    static void Main()
    {
        var solver = new Program();
        solver.Solve();
    }
}