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(); }