class Program
{
    static void Main(string[] args)
    {
        string[] inputNum = Console.ReadLine()!.Split(' ');
        int allLength = int.Parse(inputNum[0]);
        int once = int.Parse(inputNum[1]);

        if (once * 2 == allLength)
        {
            Console.WriteLine(0);
        }
        else
        {
            int half = allLength / 2;
            int num = half / once;
            int ans = num * once;

            if (ans == half)
            {
                ans -= 1;
            }

            Console.WriteLine(ans);
        }
    }
}