import java.util.Scanner;

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

		int N = sc.nextInt();

		double s = 0;
		for(int i = 0;i < N;i++) {
			int a = sc.nextInt();
			s += 1.0/a;
		}

		sc.close();


		System.out.println(1/s);

	}
} 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 s = 0;
		for(int i = 0;i < N;i++) {
			int a = sc.nextInt();
			s += 1.0/a;
		}

		sc.close();


		System.out.println(1/s);

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

		int N = sc.nextInt();

		double s = 0;
		for(int i = 0;i < N;i++) {
			int a = sc.nextInt();
			s += 1.0/a;
		}

		sc.close();


		System.out.println(1/s);

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

		int N = sc.nextInt();

		double s = 0;
		for(int i = 0;i < N;i++) {
			int a = sc.nextInt();
			s += 1.0/a;
		}

		sc.close();


		System.out.println(1/s);

	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc double s = 0; s 0 for(int i = 0;i < N;i++) {
			int a = sc.nextInt();
			s += 1.0/a;
		} int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
			int a = sc.nextInt();
			s += 1.0/a;
		} {
			int a = sc.nextInt();
			s += 1.0/a;
		} int a = sc.nextInt(); a sc.nextInt() sc.nextInt sc s += 1.0/a s 1.0/a 1.0 a sc.close() sc.close sc System.out.println(1/s) System.out.println System.out System System.out 1/s 1 s String[] args args public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int N = sc.nextInt();

		double s = 0;
		for(int i = 0;i < N;i++) {
			int a = sc.nextInt();
			s += 1.0/a;
		}

		sc.close();


		System.out.println(1/s);

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

		int N = sc.nextInt();

		double s = 0;
		for(int i = 0;i < N;i++) {
			int a = sc.nextInt();
			s += 1.0/a;
		}

		sc.close();


		System.out.println(1/s);

	}
} 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 s = 0;
		for(int i = 0;i < N;i++) {
			int a = sc.nextInt();
			s += 1.0/a;
		}

		sc.close();


		System.out.println(1/s);

	}
}
