object Main {
  def main(args: Array[String]): Unit = {
    val sc = new java.util.Scanner(System.in)
    val k = sc.nextInt
    val ra =
      (0 until k).foldLeft(Rat(0, 1))((acc, _) => acc + Rat(1, sc.nextInt))
    sc.close()

    println(BigDecimal(ra.d) / BigDecimal(ra.n))
  }

  object Rat {
    def apply(a: BigInt, b: BigInt): Rat = {
      if (b == 0) throw new ArithmeticException
      val g = a gcd b
      val n = a * b.signum / g
      val d = b * b.signum / g
      new Rat(n, d)
    }
  }

  class Rat private (val n: BigInt, val d: BigInt) {
    def +(that: Rat): Rat = Rat(n * that.d + that.n * d, d * that.d)
  }
}