use std::{f64::consts::PI, io::Read};

fn main() {
	let mut s = String::new();
	std::io::stdin().read_to_string(&mut s).ok();
	let n: Vec<f64> = s.split_whitespace().flat_map(str::parse).collect();
	println!(
		"{}",
		n[0] * (n[1] + n[2]) * ((n[2] - n[1]) * PI).powi(2) / 4.
	)
}