import java.util.*

fun main(args: Array<String>) {
    val k = Integer.parseInt(readNext())
    val s = readNext()
    val list = s.split(" ")
    var doubleList = ArrayList<Double>()
    for(i in 0..(k - 1)) {
        doubleList.add(1.0 / list.get(i).toDouble())
    }
    val sum = doubleList.sum()
    val result = 1 / sum
    println(result)
//    println(result)
}


    fun readNext(): String {
        return readLine()!!
    }
