import java.math.BigDecimal

fun main(args: Array<String>) {
    readLine()!!
    val al = readLine()!!.split(" ").map { BigDecimal(it) }

    val mo: BigDecimal = al.reduce { a, b -> a.multiply(b) }
    val ch = al.map { mo.divide(it) }.reduce { a, b -> a.add(b) }

    print(mo.divide(ch, 6, BigDecimal.ROUND_HALF_UP))
}
