package yukiCoder;

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

public class No00063 {

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

		String[] input = new String[2];

		try {
			input = bufferedReader.readLine().split(" ");

			long length = Long.parseLong(input[0]);
			int eat = Integer.parseInt(input[1]);

			int ans = calculation(length,eat);

			System.out.println(ans);
		} catch (IOException e) {
			e.printStackTrace();
		}
	}

	public static int calculation(long length, int eat) {
		int cnt = 0;
		cnt = (int) (length/(eat*2));
		if (length%(eat*2) == 0) {
			cnt--;
		}
		return cnt * eat;
	}

}