using System;
using System.Linq;

class Sample
{
    public static void Main()
    {
        var pokky = Console.ReadLine().Trim().Split(" ")
            .Select(int.Parse).ToList();
        Console.WriteLine(pokky[0]/(2*pokky[1])==0?pokky[0]/2: pokky[1]*(pokky[0] / (2*pokky[1])));
    }
}