using System;

public class Hello
{
    public static void Main()
    {
        string[] line = Console.ReadLine().Trim().Split(' ');
        var n = int.Parse(line[0]);
        var k = int.Parse(line[1]);
        int ans;
        if (k == 0 | k > n) ans = 0;
        else if (k == (n + 1) / 2) ans = n - 1;
        else ans = n - 2;
        Console.WriteLine(ans);
    }
}