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;
        let pp = p * p;
        let mut v = 0f64;
        let m = 17;
        let mut de = -1.0 / (1..=m).fold(1f64, |s, a| s * a as f64);
        for i in 0..=7 {
            v = v * pp + de;
            de = -de * ((m - 2 * i) * (m - 2 * i - 1)) as f64;
        }
        v * r * r * pp * p
    };
    let n = 1 << 20;
    let f = 1f64 / n as f64;
    let mut sum = 0f64;
    for i in (0..n).rev() {
        let l = (n - 1 - i) as f64 * f;
        sum += func(l);
    }
    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();
}