fn calc(d: f64) -> f64 {
    let func = |r: f64| -> f64 {
        if 2.0 * r < d {
            return 0f64;
        }
        let p = (d / (2.0 * r)).acos() * 2.0;
        r * r * (p - p.sin())
    };
    let n = 1 << 12;
    let f = 1f64 / n as f64;
    let inv6 = 1.0 / 6.0;
    let mut sum = 0f64;
    for i in (0..n).rev() {
        let r = (n - i) as f64 * f;
        let l = (n - 1 - i) as f64 * f;
        sum += (func(l) + 4.0 * func((l + r) * 0.5) + func(r)) * inv6;
    }
    sum * f
}

fn run() {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).unwrap();
    let mut it = s.trim().split_whitespace();
    let r: f64 = it.next().unwrap().parse().unwrap();
    let h: f64 = it.next().unwrap().parse().unwrap();
    let d: f64 = it.next().unwrap().parse().unwrap();
    let ans = r * r * h * calc(d / r);
    println!("{:.7}", ans);
}

fn main() {
    run();
}