fn main() {
	let mut rc = String::new();
	std::io::stdin().read_line(&mut rc).ok();
	let mut rc = rc.trim().split_whitespace();
	let r: u64 = rc.next().unwrap().parse().unwrap();
	let c: u64 = rc.next().unwrap().parse().unwrap();

	if r != c {
		if r % 2 == 0 || c % 2 == 0 {
			println!("{}", r * c / 2 - 1);
		} else {
			println!("{}", r * c / 2);
		}
	} else {
		if r % 2 == 0 {
			println!("{}", r * c / 4 -1);
		} else {
			println!("{}", r * c / 4);
		}
	}
}