import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		double denom = 0;
		int n = scanner.nextInt();
		for (int i = 0, a; i < n; i++) {
			a = scanner.nextInt();
			denom += (double) 1 / a;
		}
		System.out.println(1 / denom);

	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		double denom = 0;
		int n = scanner.nextInt();
		for (int i = 0, a; i < n; i++) {
			a = scanner.nextInt();
			denom += (double) 1 / a;
		}
		System.out.println(1 / denom);

	}
} Main public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		double denom = 0;
		int n = scanner.nextInt();
		for (int i = 0, a; i < n; i++) {
			a = scanner.nextInt();
			denom += (double) 1 / a;
		}
		System.out.println(1 / denom);

	} main {
		Scanner scanner = new Scanner(System.in);
		double denom = 0;
		int n = scanner.nextInt();
		for (int i = 0, a; i < n; i++) {
			a = scanner.nextInt();
			denom += (double) 1 / a;
		}
		System.out.println(1 / denom);

	} Scanner scanner = new Scanner(System.in); scanner new Scanner(System.in) double denom = 0; denom 0 int n = scanner.nextInt(); n scanner.nextInt() scanner.nextInt scanner for (int i = 0, a; i < n; i++) {
			a = scanner.nextInt();
			denom += (double) 1 / a;
		} int i = 0, a; int i = 0 i 0 a; a i < n i n i++ i++ i {
			a = scanner.nextInt();
			denom += (double) 1 / a;
		} {
			a = scanner.nextInt();
			denom += (double) 1 / a;
		} a = scanner.nextInt() a scanner.nextInt() scanner.nextInt scanner denom += (double) 1 / a denom (double) 1 / a (double) 1 a System.out.println(1 / denom) System.out.println System.out System System.out 1 / denom 1 denom String[] args args public class Main {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		double denom = 0;
		int n = scanner.nextInt();
		for (int i = 0, a; i < n; i++) {
			a = scanner.nextInt();
			denom += (double) 1 / a;
		}
		System.out.println(1 / denom);

	}
} public class Main {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		double denom = 0;
		int n = scanner.nextInt();
		for (int i = 0, a; i < n; i++) {
			a = scanner.nextInt();
			denom += (double) 1 / a;
		}
		System.out.println(1 / denom);

	}
} Main
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		double denom = 0;
		int n = scanner.nextInt();
		for (int i = 0, a; i < n; i++) {
			a = scanner.nextInt();
			denom += (double) 1 / a;
		}
		System.out.println(1 / denom);

	}
}