using System;
namespace yukicoder
{
    class Program
    {

        static void Main(string[] args)
        {
            var L = Console.ReadLine();
            var z = L.Split(" ");
            int[] ints = new int[z.Length];
            //文字分割
            for (int i = 0; i < z.Length; i++)
            {
                ints[i] = int.Parse(z[i]);
            }

            if (ints[0] / 2 <= ints[1])
            {
                Console.WriteLine("0");
            }
            else if ((ints[0] % ints[1] == 0))
            {
                int temp = ints[0] / (ints[1] * 2);
                int ans = temp * ints[1] - ints[1];
                Console.WriteLine(ans);
            }
            else 
            {
                int temp = ints[0] / (ints[1] * 2);
                int ans = temp * ints[1];
                Console.WriteLine(ans);
            }
        }
    }
}