import java.util.*
 
fun main(args: Array<String>) {
  var a = readLine()!!.toInt()-1
  var num = readLine()!!.split(" ").map{ it -> it.toFloat() }.toMutableList()
  var counter = 0f
  
  for (i in 0..a) {
    counter = counter + (1/num[i])
  }
  println((1/counter).toFloat())
}