using System;

namespace AtCoder
{
    class Program
    {
        static void Main()
        {
            var inputs = Console.ReadLine().Split(' ');
            int L = int.Parse(inputs[0]);
            int K = int.Parse(inputs[1]);
            int temp = L / (2 * K);
            int count = L % (2 * K) == 0 ? temp - 1 : temp;
            Console.WriteLine(count * K);
        }
    }
}