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

namespace Competitive_Programming
{
    class MainClass
    {
        Scanner sc;
        static void Main(string[] args)
        {
            new MainClass().solve();
        }

        void solve()
        {
            sc = new Scanner();
            int N = sc.NextInt();
            int K = sc.NextInt();
            int ans;
            if (K == 0 || K > N)
            {
                ans = 0;
            }
            else
            {
                if (N % 2 == 1 && (N + 1) / 2 == K)
                {
                    ans = N - 1;
                }
                else
                {
                    ans = N - 2;
                }
            }
            Console.WriteLine(ans);
        }
    }
    
    class Scanner
    {
        Queue<string> buf;
        public Scanner()
        {
            buf = new Queue<string>();
            this.GetBuf();
        }
        private void GetBuf()
        {
            if (buf.Count == 0)
            {
                string[] stringArray = Console.ReadLine().Split(new char[] { ' ' });
                foreach (string e in stringArray)
                {
                    buf.Enqueue(e);
                }
            }
        }
        public int NextInt()
        {
            this.GetBuf();
            return int.Parse(buf.Dequeue());
        }
        public long NextLong()
        {
            this.GetBuf();
            return long.Parse(buf.Dequeue());
        }
        public double NextDouble()
        {
            this.GetBuf();
            return double.Parse(buf.Dequeue());
        }
        public string Next()
        {
            this.GetBuf();
            return buf.Dequeue();
        }
    }
}