let () = Scanf.scanf "%d" @@ fun n ->
  let aa = Array.init n @@ fun _ -> Scanf.scanf " %d" Fun.id in
  let ans = ref 0.0 in
  for i = 0 to n - 1 do
    ans := !ans +. 1.0 /. float_of_int aa.(i)
  done;
  Printf.printf "%f\n" (1.0 /. !ans)