let pi = 4. *. atan 1.

let v r d = 2. *. pi *. pi *. r *. r *. d

let () =
  let c, ri, ro = Scanf.scanf "%d %d %d " (fun c ri ro -> c, ri, ro) in
  let c, ri, ro = float c, float ri, float ro in
  let r = (ri -. ro) /. 2. in
  let d = (ri +. ro) /. 2. in
  Printf.printf "%.10f\n" (c *. v r d)