import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		long R = scan.nextLong();
		long C = scan.nextLong();
		scan.close();
		if(R != C) {
			if(R % 2 == 1 && C % 2 == 1) {
				System.out.println((R * C) / 2);
			}else {
				System.out.println((R * C - 1) / 2);
			}
		}else {
			if(R % 2 == 0) {
				System.out.println(R * C / 4 - 1);
			}else {
				System.out.println((R * C - 1) / 4);
			}
		}
	}
}