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

class Program
{
    public static void Main(string[] args)
    {
        var line = Console.ReadLine().Split(' ').Select(long.Parse).ToArray();
        var l = line[0];
        var k = line[1];
        bool yu = false;
        long sum = 0;
        while (l > k)
        {
            l -= k;
            if (yu)
            {
                sum += k;
            }
            yu = !yu;
        }
        Console.WriteLine(sum);
    }    
}