fn main() {
    let mut abc = String::new();
    std::io::stdin().read_line(&mut abc).ok();
    let abc: Vec<f64> = abc.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
    let a = abc[0];
    let b = abc[1];
    let c = abc[2];

    let s = abc.iter().sum::<f64>() / 2.;
    let area = s * (s - a) * (s - b) * (s - c);
    let area = area.sqrt();
    println!("{}" , area/4.);
}