(* --------------Encoder-------------- *)
(* -----function----- *)
let encoder = float_of_string (read_line ())

(* -------test------- *)
(*let test1 = (encoder = 0.0)*)
(* ----------------------------------- *)

(* --------------Decoder-------------- *)
(* -----function----- *)
let decoder ans = string_of_float ans

(* -------test------- *)
(* ----------------------------------- *)

(* --------------Processor-------------- *)
(* -----function----- *)
(* purpose
 *      与えられた半径から周長を求める
*)

(* function type
        processor : float -> float
*)
let rec processor r = 2.0 *. Float.pi *. r
(* -------test------- *)
let test1 = (processor 0.0 = 0.0)
let test2 = (processor 1.0 = 6.28318530717958623200)
(* ----------------------------------- *)


(* ----------------main---------------- *)
(*let () = print_endline (decoder (processor encoder))*)
let main () =
        Printf.fprintf stdout "%.20f\n" (processor encoder)
let _ = main ()
(* ------------------------------------ *)
