import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Double d = 0.0;
		Scanner scan = new Scanner(System.in);
		int a = Integer.parseInt(scan.nextLine());
		for (int i = 0; i < a; i++) {
			d += 1 / scan.nextDouble();
		}
		System.out.println(1 / d);
		scan.close();
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {
		Double d = 0.0;
		Scanner scan = new Scanner(System.in);
		int a = Integer.parseInt(scan.nextLine());
		for (int i = 0; i < a; i++) {
			d += 1 / scan.nextDouble();
		}
		System.out.println(1 / d);
		scan.close();
	}
} Main public static void main(String[] args) {
		Double d = 0.0;
		Scanner scan = new Scanner(System.in);
		int a = Integer.parseInt(scan.nextLine());
		for (int i = 0; i < a; i++) {
			d += 1 / scan.nextDouble();
		}
		System.out.println(1 / d);
		scan.close();
	} main {
		Double d = 0.0;
		Scanner scan = new Scanner(System.in);
		int a = Integer.parseInt(scan.nextLine());
		for (int i = 0; i < a; i++) {
			d += 1 / scan.nextDouble();
		}
		System.out.println(1 / d);
		scan.close();
	} Double d = 0.0; d 0.0 Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int a = Integer.parseInt(scan.nextLine()); a Integer.parseInt(scan.nextLine()) Integer.parseInt Integer scan.nextLine() scan.nextLine scan for (int i = 0; i < a; i++) {
			d += 1 / scan.nextDouble();
		} int i = 0; int i = 0; i 0 i < a i a i++ i++ i {
			d += 1 / scan.nextDouble();
		} {
			d += 1 / scan.nextDouble();
		} d += 1 / scan.nextDouble() d 1 / scan.nextDouble() 1 scan.nextDouble() scan.nextDouble scan System.out.println(1 / d) System.out.println System.out System System.out 1 / d 1 d scan.close() scan.close scan String[] args args public class Main {
	public static void main(String[] args) {
		Double d = 0.0;
		Scanner scan = new Scanner(System.in);
		int a = Integer.parseInt(scan.nextLine());
		for (int i = 0; i < a; i++) {
			d += 1 / scan.nextDouble();
		}
		System.out.println(1 / d);
		scan.close();
	}
} public class Main {
	public static void main(String[] args) {
		Double d = 0.0;
		Scanner scan = new Scanner(System.in);
		int a = Integer.parseInt(scan.nextLine());
		for (int i = 0; i < a; i++) {
			d += 1 / scan.nextDouble();
		}
		System.out.println(1 / d);
		scan.close();
	}
} Main
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Double d = 0.0;
		Scanner scan = new Scanner(System.in);
		int a = Integer.parseInt(scan.nextLine());
		for (int i = 0; i < a; i++) {
			d += 1 / scan.nextDouble();
		}
		System.out.println(1 / d);
		scan.close();
	}
}