using System;

class Program
{
    static void Main(string[] args)
    {
        String[] input = Console.ReadLine().Split(' ');

        double a = double.Parse(input[0]) / double.Parse(input[1]) / 2;
        double b = Math.Floor(a);
        if (a == b) { b--; }

        Console.WriteLine(a == 1 ? 0 : int.Parse(input[1]) * b);
    }
}