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 N = scan.nextInt();

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

		for(int i = 0; i < N; i++) {
			A.add(scan.nextDouble());
		}

		double ans = 0;
		for(int i = 0; i < N; i++) {
			ans += (1/A.get(i));
		}

		System.out.println(1/ans);
	}
} 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 N = scan.nextInt();

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

		for(int i = 0; i < N; i++) {
			A.add(scan.nextDouble());
		}

		double ans = 0;
		for(int i = 0; i < N; i++) {
			ans += (1/A.get(i));
		}

		System.out.println(1/ans);
	}
} Main public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		int N = scan.nextInt();

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

		for(int i = 0; i < N; i++) {
			A.add(scan.nextDouble());
		}

		double ans = 0;
		for(int i = 0; i < N; i++) {
			ans += (1/A.get(i));
		}

		System.out.println(1/ans);
	} main {

		Scanner scan = new Scanner(System.in);

		int N = scan.nextInt();

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

		for(int i = 0; i < N; i++) {
			A.add(scan.nextDouble());
		}

		double ans = 0;
		for(int i = 0; i < N; i++) {
			ans += (1/A.get(i));
		}

		System.out.println(1/ans);
	} Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int N = scan.nextInt(); N scan.nextInt() scan.nextInt scan List<Double> A = new ArrayList<>(); A new ArrayList<>() for(int i = 0; i < N; i++) {
			A.add(scan.nextDouble());
		} int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
			A.add(scan.nextDouble());
		} {
			A.add(scan.nextDouble());
		} A.add(scan.nextDouble()) A.add A scan.nextDouble() scan.nextDouble scan double ans = 0; ans 0 for(int i = 0; i < N; i++) {
			ans += (1/A.get(i));
		} int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
			ans += (1/A.get(i));
		} {
			ans += (1/A.get(i));
		} ans += (1/A.get(i)) ans (1/A.get(i)) 1 A.get(i) A.get A i System.out.println(1/ans) System.out.println System.out System System.out 1/ans 1 ans String[] args args public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		int N = scan.nextInt();

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

		for(int i = 0; i < N; i++) {
			A.add(scan.nextDouble());
		}

		double ans = 0;
		for(int i = 0; i < N; i++) {
			ans += (1/A.get(i));
		}

		System.out.println(1/ans);
	}
} public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		int N = scan.nextInt();

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

		for(int i = 0; i < N; i++) {
			A.add(scan.nextDouble());
		}

		double ans = 0;
		for(int i = 0; i < N; i++) {
			ans += (1/A.get(i));
		}

		System.out.println(1/ans);
	}
} 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 N = scan.nextInt();

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

		for(int i = 0; i < N; i++) {
			A.add(scan.nextDouble());
		}

		double ans = 0;
		for(int i = 0; i < N; i++) {
			ans += (1/A.get(i));
		}

		System.out.println(1/ans);
	}
}