import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.stream.Stream;

public class No00000063_Main {

	public static void main(String[] args) throws IOException {

		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        int[] arr = Stream.of(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray();
        int i = arr[0] / arr[1] == 0 && (arr[0] / arr[1]) %  2 == 0 ? 1 : 0;
		System.out.println(arr[0] <= 2 * arr[1] ? 0 : (((arr[0] / arr[1]) / 2 - i) * arr[1]));

	}

}