use std::io::*;
use std::str::FromStr;

#[allow(dead_code)]
fn get_line() -> String {
    let stdin = stdin();
    let mut line = String::new();
    stdin.lock().read_line(&mut line).expect("io error.");
    line.trim().to_string()
}

#[allow(dead_code)]
fn cast<T: FromStr>(s: &str) -> T {
    s.parse().ok().expect("parse error.")
}

#[allow(dead_code)]
fn get_vec<T: FromStr>() -> Vec<T> {
    (&get_line()).split(' ').map(cast::<T>).collect()
}


/* ---------- ここまでテンプレ ---------- */


fn main() {
    let v: Vec<u64> = get_vec();

    let ans =
        if v[0] == v[1] {
            match v[0] % 2 {
                0 => v[0] * v[0] / 4 - 1,
                _ => (v[0] / 2) * (v[0] / 2 + 1),
            }
        } else {
            match (v[0] % 2, v[1] % 2) {
                (1, 1) => v[0] * (v[1] / 2) + v[0] / 2,
                (_, _) => v[0] * v[1] / 2 - 1,
            }
        };

    println!("{}", ans);
}