using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace yukicoder_63 {
    class Program {
        static void Main(string[] args) {
            string[] a = Console.ReadLine().Split(' ');
            int L = int.Parse(a[0]);
            int K = int.Parse(a[1]);
            int count = 0;
            for (int i= 0; L > 2 * K; i++) {
                L -= 2 * K;
                count++;
            }
            Console.WriteLine(count * K);
            Console.ReadLine();
        }
    }
}