using System;

namespace No._63
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] lines = Console.ReadLine().Split();
            int L = int.Parse(lines[0]);
            int K = int.Parse(lines[1]);

            int U = 0;

            while (true)
            {
                L -= K * 2;

                if (L <= 0) break;

                U += K;
            }

            Console.WriteLine(U);
        }
    }
}