using System;

public class Program {
	public static void Main(string[] args) {
		string[] s = Console.ReadLine().Split(' ');
		long l = long.Parse(s[0]);
		int k = int.Parse(s[1]);
		int len = 0;
		while (l > 0) {
			if (l - k * 2 > 0) {
				len += k;
			}
			l -= k * 2;
		}
		Console.WriteLine(len);
	}
}