import scala.math.ceil

object Main {
  val sc = new java.util.Scanner(System.in)

  def sin(n: Int): Seq[Int] = Seq.fill(n)(sc.nextInt())

  def iin(): Int = sc.nextLine().toInt

  def main(args: Array[String]): Unit = {
    val n = iin()
    val a = sin(n)

    println( 1.0 / a.foldLeft(0.0)((b, x) => b + 1 / x.toDouble) )
  }
}