fun main(args:Array<String>){
    val numOfInt = readLine()!!.toInt()
    val prog = readLine()!!.split(" ").map(String::toInt)
    var ans = 0.0
    for(i in 0..numOfInt-1){
        ans += 1 / prog[i].toDouble()
    }

    print(1 / ans)
}