using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

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 ans = 0;
            int end = K;

            while (L - end * 2 > 0)
            {
                ans += K;
                end = end + K;
            }

            Console.WriteLine(ans);
        }
    }
}