fn main() {
	let mut s = String::new();
	std::io::stdin().read_line(&mut s).ok();
	let mut s: Vec<f64> = s.trim().split_whitespace()
		.map(|e| e.parse().unwrap()).collect();
	let a = s[0];
	let b = s[1];
	let c = s[2];
	
	let s = (a + b + c) / 2.;
	
	let ans = (s * (s - a) * (s - b) * (s -c)).sqrt() / 4.;
	println!("{}", ans);
}