import java.util.Scanner;

class Test {
	public static void main(String[] args) throws Exception {
		Scanner in = new Scanner(System.in);
		int l = in.nextInt();
		int k = in.nextInt();

		int temp = k * 2;
		if(l % temp == 0) {
			System.out.println((l / temp - 1) * k);
		} else {
			System.out.println(l / temp * k);
		}
	}
}