open Printf
open Scanf

let () =
  let n = scanf "%d" (fun x -> x) in
  Array.init n (fun _ -> scanf " %f" (fun x -> x))
  |> Array.map (fun x -> 1.0 /. x)
  |> Array.fold_left (+.) 0.0
  |> (fun x -> 1.0 /. x)
  |> printf "%.15f\n"