using System;
using System.Text;
using System.Linq;

namespace ConsoleApp11
{
    class Program
    {
        static void Main(string[] args)
        {
            long sum = 0;

            var input = Console.ReadLine().Split(' ');

            long L = long.Parse(input[0]);
            long K = long.Parse(input[1]);

            while (true)
            {
                if(L <= K * 2)
                {
                    break;
                }

                L -= K * 2;

                sum += K;

                if(L <= K * 2)
                {
                    break;
                }
            }
            Console.WriteLine(sum);
        }
    }
}