using System;
using System.Linq;

public class P0063 {
    public static void Main() {
        var lk = Console.ReadLine().Trim().Split(' ').Select(int.Parse).ToList();
        var l = lk[0];
        var k = lk[1];
        var count = l / k / 2;
        count -= l % (2*k) == 0 ? 1 : 0;
        Console.WriteLine(count * k);
    }
}