// ---------- begin input macro ----------
// reference: https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};
    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}

macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };
    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };
    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };
    ($iter:expr, bytes) => {
        read_value!($iter, String).bytes().collect::<Vec<u8>>()
    };
    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };
    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}
// ---------- end input macro ----------

fn calc(d: f64) -> f64 {
    let mut sum = 0f64;
    let mut c = 0f64;
    let n = 1 << 25;
    let f = 1f64 / n as f64;
    for i in (0..n).rev() {
        let r = (n - i - 1) as f64 * f;
        if 2.0 * r < d {
            continue;
        }
        let p = (d / 2.0 / r).acos() * 2.0;
//        ans += r * r * (p - p.sin());
        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 * p * p + de;
            de = -de * (m - 2 * i) as f64 * (m - 2 * i - 1) as f64;
        }
        v *= r * r * p * p * p;
//        sum += v;
        let y = v - c;
        let t = sum + y;
        c = (t - sum) - y;
        sum = t;
    }
    sum * f
}

fn run() {
    input! {
        r: f64,
        h: f64,
        d: f64,
    }
    let ans = r * r * h * calc(d / r);
    println!("{:.7}", ans);
}

fn main() {
    run();
}