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

class Magatro
{
    static void Main()
    {
        int L, K;
        string[] q = Console.ReadLine().Split(' ');
        L = int.Parse(q[0]);
        K = int.Parse(q[1]);
        int cnt;
        cnt = L %(2* K) == 0 ? L / (2 * K) - 1 : L / (2 * K);
        Console.WriteLine(cnt * K);
    }
}