import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		@SuppressWarnings("resource")
		Scanner scanner = new Scanner(System.in);

		int N = scanner.nextInt();


		double number = 0;
		for (int i = 0; i < N; i++) {
			double A = scanner.nextDouble();
			number = number + ( 1 / A);
		}

		log(1 / number);

	}

	private static void log(String str) {
		System.out.println(str);
	}

	private static void log(int str) {
		System.out.println(str);
	}

	private static void log(double str) {
		System.out.println(str);
	}

} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {
		@SuppressWarnings("resource")
		Scanner scanner = new Scanner(System.in);

		int N = scanner.nextInt();


		double number = 0;
		for (int i = 0; i < N; i++) {
			double A = scanner.nextDouble();
			number = number + ( 1 / A);
		}

		log(1 / number);

	}

	private static void log(String str) {
		System.out.println(str);
	}

	private static void log(int str) {
		System.out.println(str);
	}

	private static void log(double str) {
		System.out.println(str);
	}

} Main public static void main(String[] args) {
		@SuppressWarnings("resource")
		Scanner scanner = new Scanner(System.in);

		int N = scanner.nextInt();


		double number = 0;
		for (int i = 0; i < N; i++) {
			double A = scanner.nextDouble();
			number = number + ( 1 / A);
		}

		log(1 / number);

	} main {
		@SuppressWarnings("resource")
		Scanner scanner = new Scanner(System.in);

		int N = scanner.nextInt();


		double number = 0;
		for (int i = 0; i < N; i++) {
			double A = scanner.nextDouble();
			number = number + ( 1 / A);
		}

		log(1 / number);

	} @SuppressWarnings("resource")
		Scanner scanner = new Scanner(System.in); scanner new Scanner(System.in) int N = scanner.nextInt(); N scanner.nextInt() scanner.nextInt scanner double number = 0; number 0 for (int i = 0; i < N; i++) {
			double A = scanner.nextDouble();
			number = number + ( 1 / A);
		} int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
			double A = scanner.nextDouble();
			number = number + ( 1 / A);
		} {
			double A = scanner.nextDouble();
			number = number + ( 1 / A);
		} double A = scanner.nextDouble(); A scanner.nextDouble() scanner.nextDouble scanner number = number + ( 1 / A) number number + ( 1 / A) number ( 1 / A) 1 A log(1 / number) log 1 / number 1 number String[] args args private static void log(String str) {
		System.out.println(str);
	} log {
		System.out.println(str);
	} System.out.println(str) System.out.println System.out System System.out str String str str private static void log(int str) {
		System.out.println(str);
	} log {
		System.out.println(str);
	} System.out.println(str) System.out.println System.out System System.out str int str str private static void log(double str) {
		System.out.println(str);
	} log {
		System.out.println(str);
	} System.out.println(str) System.out.println System.out System System.out str double str str public class Main {
	public static void main(String[] args) {
		@SuppressWarnings("resource")
		Scanner scanner = new Scanner(System.in);

		int N = scanner.nextInt();


		double number = 0;
		for (int i = 0; i < N; i++) {
			double A = scanner.nextDouble();
			number = number + ( 1 / A);
		}

		log(1 / number);

	}

	private static void log(String str) {
		System.out.println(str);
	}

	private static void log(int str) {
		System.out.println(str);
	}

	private static void log(double str) {
		System.out.println(str);
	}

} public class Main {
	public static void main(String[] args) {
		@SuppressWarnings("resource")
		Scanner scanner = new Scanner(System.in);

		int N = scanner.nextInt();


		double number = 0;
		for (int i = 0; i < N; i++) {
			double A = scanner.nextDouble();
			number = number + ( 1 / A);
		}

		log(1 / number);

	}

	private static void log(String str) {
		System.out.println(str);
	}

	private static void log(int str) {
		System.out.println(str);
	}

	private static void log(double str) {
		System.out.println(str);
	}

} Main
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		@SuppressWarnings("resource")
		Scanner scanner = new Scanner(System.in);

		int N = scanner.nextInt();


		double number = 0;
		for (int i = 0; i < N; i++) {
			double A = scanner.nextDouble();
			number = number + ( 1 / A);
		}

		log(1 / number);

	}

	private static void log(String str) {
		System.out.println(str);
	}

	private static void log(int str) {
		System.out.println(str);
	}

	private static void log(double str) {
		System.out.println(str);
	}

}