import java.util.Scanner;


public class Main_yukicoder84 {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        long r = sc.nextInt();
        long c = sc.nextInt();

        long ret = (r / 2) * (c / 2);
        if (r != c) {
        	ret *= 2;
        }
        ret += (r % 2) * (c / 2);
        if (r != c) {
        	ret += (c % 2) * (r / 2);
        }
        if ((r % 2) * (c % 2) != 0) {
        	ret++;
        }

        System.out.println(ret - 1);

        sc.close();
    }
}