use std::io::*;

fn main() {
    let mut s: String = String::new();
    std::io::stdin().read_to_string(&mut s).ok();
    let mut itr = s.trim().split_whitespace();
    let r: usize = itr.next().unwrap().parse().unwrap();
    let c: usize = itr.next().unwrap().parse().unwrap();
    if r == c {
        println!("{}", (r * r - 1) / 4);
    } else {
        println!("{}", (r * c - 1) / 2);
    }
}