Public Class Program
  Shared Sub Main()
    Dim n As Integer = Integer.Parse(Console.ReadLine())
    Dim arrayA As Integer() = Array.ConvertAll(Of String, Integer)(Console.ReadLine().Split(" "), AddressOf Integer.Parse)
    Dim ans As Double = 0
    For i As Integer = 0 To n - 1
      ans += 1 / arrayA(i)
    Next
    Console.WriteLine((1 / ans).ToString())
  End Sub
End Class