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

		File f = new File("entrada");
		if (f.exists()) {
			in = new BufferedReader(new FileReader(f));
		} else
			in = new BufferedReader(new InputStreamReader(System.in));

		int dates[] = readInts(nextLine());
		int n = dates[0];
		int m = dates[1];
		if (m == 1)
			System.out.println(n / 2);
		else if (n == 1)
			System.out.println(m / 2);
		else if (n % 2 != 0 && m % 2 != 0) {
			System.out.println(((n / 2) * m) + ((m / 2)));
		} else {
			if (n % 2 == 0 && m % 2 == 0) {
				System.out.println((n / 2) * m);
			} else {
				int par = n % 2 == 0 ? n : m;
				int impar = m % 2 != 0 ? m : n;
				System.out.println(par / 2 * impar);
			}

		}

	}