import java.util.*

fun main(args: Array<String>) {
    val argList = Scanner(System.`in`)

    val n = argList.nextLine().toInt()
    val a = argList.nextLine().split(" ").map { it.toInt() }

    val e = ResistorsInParallel()
    e.execute(n, a)

}

class ResistorsInParallel {
    fun execute(n: Int, a: List<Int>) {
        val result = a.map {
            1F / it
        }

        var sumR = 0F

        result.forEach {
            sumR += it
        }
        println(1 / sumR)
    }
}