import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args) {
		BufferedReader buff = new BufferedReader(new InputStreamReader(System.in));

		try {
			String[] box = buff.readLine().split(" ");
			int L = Integer.parseInt(box[0]);
			int K = Integer.parseInt(box[1]);
			int count = 0;
			while(L - K * 2 > 0){
				count++;
				L -= K *2;
			}
			System.out.println(count * K);
		} catch (NumberFormatException e) {

		} catch (IOException e) {

		}
	}
}