using System;
using System.Linq;

namespace No63
{
    class Program
    {
        static void Main(string[] args)
        {
            var LK = Console.ReadLine().Split(' ').Select(i => double.Parse(i)).ToArray();
            var L = LK[0];
            var K = LK[1];

            var result = Math.Max(
                Math.Ceiling(L / (2 * K)) - 1d,
                0) * K;
            Console.Write(result);
        }
    }
}