fun main(args: Array<String>)
{
    val n = readInt()
    val a = readNInts()

    val invA = mutableListOf<Double>()
    for (i in 0..n-1) {
        invA.add(1.0/a[i].toDouble())
    }

    var sum = 0.0
    for (item in invA) {
        sum += item
    }

    println(1.0 / sum)

    // val n = readInt()
    // val a = readNInts()

    // val prod = mutableListOf<Double>()
    // for (i in 1..n) {
    //     var tmp = 1.0
    //     for (excluded in 1..n) {
    //         tmp *= if (i == excluded) 1.0 else a[i-1].toDouble()
    //     }
    //     prod.add(tmp)
    // }
    // var allProd = 1.0
    // for (item in a) {
    //     allProd *= item.toDouble()
    // }

    // var sum = 0.0
    // for (item in prod) {
    //     sum += item
    // }

    // println(if (n == 1) a[0] else allProd/sum)

}


// Read an integer
// Usage: val a = readInt()
private fun readInt() = readLine()!!.toInt()

// Read N integers separated by " "
// Usage: val (a, b, c) = readNInts()
private fun readNInts() = readLine()!!.split(" ").map(String::toInt)

// Read a string
// Usage: val a = readString()
private fun readString() = readLine()!!
