using System;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        int[] LK = Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();
        int yuu = 0;

        while (0 < LK[0] - LK[1] * 2)
        {
            yuu += LK[1];
            LK[0] -= LK[1] * 2;
        }

        Console.WriteLine(yuu);
    }
}