[<EntryPoint>]
let main argv =
    //printfn "Make it!"
    let n = System.Console.ReadLine() |> int
    let xs = System.Console.ReadLine().Split() |> Array.map float |> Array.toList
    let c = List.sumBy (fun x -> 1.0/x) xs
    let ans = 1.0 / c
    printfn "%f" ans
    0
