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

namespace YukiCoder_Cs
{
    class Program
    {
        public static void Main()
        {
            var line = Console.ReadLine().Split(' ').ToArray();
            int N = int.Parse(line[0]);
            int K = int.Parse(line[1]);

            int ans = 0;
            if (K > N || 1 > K)
            {
                ans = 0;
            }
            else if (N % 2 == 1 && (N / 2) + 1 == K)
            {
                ans = N - 1;
            }
            else
            {
                ans = N - 2;
            }

            Console.WriteLine($"{ans}");

        }
    }
}