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

class Program
{
    static void Main(string[] args)
    {
        new Magatro().Solve();
    }
}

class Magatro
{
    private int N, K;
    public void Solve()
    {
        var line = Console.ReadLine().Split(' ');
        N = int.Parse(line[0]);
        K = int.Parse(line[1]);
        if(K == 0 || K > N)
        {
            Console.WriteLine(0);
            return;
        }
        if (N % 2 == 1 && (N + 1) / 2 == K)
        {
            Console.WriteLine(N - 1);
            return;
        }
        else
        {
            Console.WriteLine(N - 2);
            return;
        }
    }
}