use std::f64::consts::PI;

fn main() {
    let mut input = String::new();
    std::io::Read::read_to_string(&mut std::io::stdin(), &mut input).ok();
    let input: Vec<f64> = input
        .split_whitespace()
        .map(|n| n.parse().unwrap())
        .collect();

    let r = (input[2] - input[1]) / 2.;
    let answer = input[0] * (2. * PI * (input[1] + r)) * (PI * r.powi(2));

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