using System;
using System.Collections.Generic;
using System.Linq;
using static System.Console;

class Program
{
    static void Main()
    {
        var lk = ReadLine().Split().Select(int.Parse).ToArray();
        WriteLine((lk[0] / (lk[1] * 2) + (lk[0]) % (lk[1] * 2) == 0 ? -1 : 0) * lk[0]);
    }
}