import java.util.Scanner;

class Main {
	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

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

		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();
		double b = 0;
		for (int i = 0; i < n; i++) {
			double a = sc.nextDouble();
			b += 1 / a;
		}
		System.out.println(1 / b);
	}
} Main public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();
		double b = 0;
		for (int i = 0; i < n; i++) {
			double a = sc.nextDouble();
			b += 1 / a;
		}
		System.out.println(1 / b);
	} main {

		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();
		double b = 0;
		for (int i = 0; i < n; i++) {
			double a = sc.nextDouble();
			b += 1 / a;
		}
		System.out.println(1 / b);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc double b = 0; b 0 for (int i = 0; i < n; i++) {
			double a = sc.nextDouble();
			b += 1 / a;
		} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
			double a = sc.nextDouble();
			b += 1 / a;
		} {
			double a = sc.nextDouble();
			b += 1 / a;
		} double a = sc.nextDouble(); a sc.nextDouble() sc.nextDouble sc b += 1 / a b 1 / a 1 a System.out.println(1 / b) System.out.println System.out System System.out 1 / b 1 b String[] args args
import java.util.Scanner;

class Main {
	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();
		double b = 0;
		for (int i = 0; i < n; i++) {
			double a = sc.nextDouble();
			b += 1 / a;
		}
		System.out.println(1 / b);
	}
}