
import java.util.Scanner

import scala.collection.immutable


object Main {

  def read(): (Int, IndexedSeq[Int]) = {
    val sc: Scanner = new Scanner(System.in)
    val n = sc.nextInt
    val arr = (1 to n).map { _ =>
      sc.nextInt
    }
    sc.close()
    (n, arr)
  }


  def solve(n: Int, arr: IndexedSeq[Int]): Double = {
    val reversedNums = for(i <- arr) yield {
      1.toDouble / i
    }
    1 / reversedNums.sum
  }


  def main(args: Array[String]): Unit = {
    val (a, s) = read()
    println(solve(a, s))
  }

}
