object Main{
  def main(args: Array[String]) = {
    val N = io.StdIn.readLine().toLong
    val a = io.StdIn.readLine().split(' ').map(_.toDouble)

    val bun = a.map{
      ai =>
        1.0 / ai
    }.sum

    println(1.0 / bun)
  }
}