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

namespace yukicoder
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = Console.ReadLine();
            string[] t = s.Split(' ');
            int L = int.Parse(t[0]);
            int K = int.Parse(t[1]);
            int haruka = 0;
            int c = L;
            while(0 < c)
            {
                if(c - (K * 2) <= 0)
                {
                    haruka += c;
                    c = 0;
                }
                else
                {
                    c -= (K * 2);
                    haruka += K;
                }
            }

            Console.WriteLine("{0}", (L - haruka));
            //Console.ReadLine();

        }
    }
}