using System;
using System.Collections.Generic;
using System.Linq;

namespace lecture
{
    class MainClass
    {
        public static void Main(string[] args)
        {

            int[] tmp = Console.ReadLine().Trim().Split(' ').Select(x => int.Parse(x)).ToArray();
            int L = tmp[0];
            int K = tmp[1];

            int count = 0;
            while(L > 0)
            {
                L -= 2 * K;
                count += K;
            }
            if (L/2 < count)
            {
                count -= K;
            }
            
            Console.WriteLine(count);
        }
    }
}