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

public class Program
{

    public void Proc() {
        long[] inpt = Reader.ReadLine().Split(' ').Select(a => long.Parse(a)).ToArray();

        int boxCount = 0;

        long ans = 0;
        if(inpt[1]==0||inpt[1]>inpt[0]) {
            ans = inpt[0];
        } else if(inpt[0]%2 == 1 && inpt[1] == (inpt[0]+1)/2) {
            ans = inpt[0] - 1;
        } else {
            ans = Math.Max(inpt[0] - 2, 0);
        }

        Console.WriteLine((ans));
    }



    public class Reader
    {
        private static StringReader sr;
        public static bool IsDebug = false;
        public static string ReadLine() {
            if(IsDebug) {
                if(sr == null) {
                    sr = new StringReader(InputText.Trim());
                }
                return sr.ReadLine();
            } else {
                return Console.ReadLine();
            }
        }
        private static string InputText = @"

5 2



";
    }

    public static void Main(string[] args)
    {
#if DEBUG
        Reader.IsDebug = true;
#endif
        Program prg = new Program();
        prg.Proc();
	}
}