[<AutoOpen>]
module Program

open System

let read f = stdin.ReadLine() |> f
let reada f = stdin.ReadLine().Split() |> Array.map f
let reads f = stdin.ReadLine().Split() |> Array.toList |> List.map f

[<EntryPoint>]
let main _ =
    let n = read int
    let xs = reads float
    List.sumBy (fun x -> 1. / x) xs
    |> fun x -> 1. / x
    |> printfn "%.6f"
    0