(* abc138 b problem *)
open Scanf;;
open Array;;

let () = Scanf.scanf "%d"
           (fun n -> Array.init n (fun _ -> Scanf.scanf " %f" (fun a -> a)))
         |> Array.fold_left (fun x a -> x +. (1.0 /. a)) 0.0
         |> ((/.) 1.0)
         |> print_float
         |> print_newline;;

exit 0;;