import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int N = sc.nextInt();

		double firstCalc = 0.0;
		for(int i = 0; i < N; i++)
		{
			firstCalc += reverseNumber(sc.nextInt());
		}

		System.out.println(String.format("%f", 1/firstCalc));

		sc.close();
	}

	private static double reverseNumber(int i) {
		return 1 / (double)i;
	}

} import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int N = sc.nextInt();

		double firstCalc = 0.0;
		for(int i = 0; i < N; i++)
		{
			firstCalc += reverseNumber(sc.nextInt());
		}

		System.out.println(String.format("%f", 1/firstCalc));

		sc.close();
	}

	private static double reverseNumber(int i) {
		return 1 / (double)i;
	}

} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int N = sc.nextInt();

		double firstCalc = 0.0;
		for(int i = 0; i < N; i++)
		{
			firstCalc += reverseNumber(sc.nextInt());
		}

		System.out.println(String.format("%f", 1/firstCalc));

		sc.close();
	} main {
		Scanner sc = new Scanner(System.in);

		int N = sc.nextInt();

		double firstCalc = 0.0;
		for(int i = 0; i < N; i++)
		{
			firstCalc += reverseNumber(sc.nextInt());
		}

		System.out.println(String.format("%f", 1/firstCalc));

		sc.close();
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc double firstCalc = 0.0; firstCalc 0.0 for(int i = 0; i < N; i++)
		{
			firstCalc += reverseNumber(sc.nextInt());
		} int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
			firstCalc += reverseNumber(sc.nextInt());
		} {
			firstCalc += reverseNumber(sc.nextInt());
		} firstCalc += reverseNumber(sc.nextInt()) firstCalc reverseNumber(sc.nextInt()) reverseNumber sc.nextInt() sc.nextInt sc System.out.println(String.format("%f", 1/firstCalc)) System.out.println System.out System System.out String.format("%f", 1/firstCalc) String.format String "%f" 1/firstCalc 1 firstCalc sc.close() sc.close sc String[] args args private static double reverseNumber(int i) {
		return 1 / (double)i;
	} reverseNumber {
		return 1 / (double)i;
	} return 1 / (double)i; 1 / (double)i 1 (double)i int i i public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int N = sc.nextInt();

		double firstCalc = 0.0;
		for(int i = 0; i < N; i++)
		{
			firstCalc += reverseNumber(sc.nextInt());
		}

		System.out.println(String.format("%f", 1/firstCalc));

		sc.close();
	}

	private static double reverseNumber(int i) {
		return 1 / (double)i;
	}

} public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int N = sc.nextInt();

		double firstCalc = 0.0;
		for(int i = 0; i < N; i++)
		{
			firstCalc += reverseNumber(sc.nextInt());
		}

		System.out.println(String.format("%f", 1/firstCalc));

		sc.close();
	}

	private static double reverseNumber(int i) {
		return 1 / (double)i;
	}

} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int N = sc.nextInt();

		double firstCalc = 0.0;
		for(int i = 0; i < N; i++)
		{
			firstCalc += reverseNumber(sc.nextInt());
		}

		System.out.println(String.format("%f", 1/firstCalc));

		sc.close();
	}

	private static double reverseNumber(int i) {
		return 1 / (double)i;
	}

}
