import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long r = sc.nextInt();
		long c = sc.nextInt();
		if (r != c) {
			System.out.println(((r * c) + 1) / 2 - 1);
		}else{
			System.out.println(((r * c) + 3) / 4 - 1);
		}
	}

}