import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		int numN = scan.nextInt();

		List<Double> numA = new ArrayList<>();

		for (int i = 0; i < numN; i++) {
			numA.add(1.0 / scan.nextDouble());
		}

		double sum = 0.0;

		for (double num: numA) {
			sum += num;
		}

		System.out.println(1.0 / sum);
	}
} import java.util.ArrayList; ArrayList java.util import java.util.List; List java.util import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		int numN = scan.nextInt();

		List<Double> numA = new ArrayList<>();

		for (int i = 0; i < numN; i++) {
			numA.add(1.0 / scan.nextDouble());
		}

		double sum = 0.0;

		for (double num: numA) {
			sum += num;
		}

		System.out.println(1.0 / sum);
	}
} Main public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		int numN = scan.nextInt();

		List<Double> numA = new ArrayList<>();

		for (int i = 0; i < numN; i++) {
			numA.add(1.0 / scan.nextDouble());
		}

		double sum = 0.0;

		for (double num: numA) {
			sum += num;
		}

		System.out.println(1.0 / sum);
	} main {

		Scanner scan = new Scanner(System.in);

		int numN = scan.nextInt();

		List<Double> numA = new ArrayList<>();

		for (int i = 0; i < numN; i++) {
			numA.add(1.0 / scan.nextDouble());
		}

		double sum = 0.0;

		for (double num: numA) {
			sum += num;
		}

		System.out.println(1.0 / sum);
	} Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int numN = scan.nextInt(); numN scan.nextInt() scan.nextInt scan List<Double> numA = new ArrayList<>(); numA new ArrayList<>() for (int i = 0; i < numN; i++) {
			numA.add(1.0 / scan.nextDouble());
		} int i = 0; int i = 0; i 0 i < numN i numN i++ i++ i {
			numA.add(1.0 / scan.nextDouble());
		} {
			numA.add(1.0 / scan.nextDouble());
		} numA.add(1.0 / scan.nextDouble()) numA.add numA 1.0 / scan.nextDouble() 1.0 scan.nextDouble() scan.nextDouble scan double sum = 0.0; sum 0.0 for (double num: numA) {
			sum += num;
		} double num numA {
			sum += num;
		} {
			sum += num;
		} sum += num sum num System.out.println(1.0 / sum) System.out.println System.out System System.out 1.0 / sum 1.0 sum String[] args args public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		int numN = scan.nextInt();

		List<Double> numA = new ArrayList<>();

		for (int i = 0; i < numN; i++) {
			numA.add(1.0 / scan.nextDouble());
		}

		double sum = 0.0;

		for (double num: numA) {
			sum += num;
		}

		System.out.println(1.0 / sum);
	}
} public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		int numN = scan.nextInt();

		List<Double> numA = new ArrayList<>();

		for (int i = 0; i < numN; i++) {
			numA.add(1.0 / scan.nextDouble());
		}

		double sum = 0.0;

		for (double num: numA) {
			sum += num;
		}

		System.out.println(1.0 / sum);
	}
} Main
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		int numN = scan.nextInt();

		List<Double> numA = new ArrayList<>();

		for (int i = 0; i < numN; i++) {
			numA.add(1.0 / scan.nextDouble());
		}

		double sum = 0.0;

		for (double num: numA) {
			sum += num;
		}

		System.out.println(1.0 / sum);
	}
}
