import java.util.Scanner;

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

public class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = Integer.parseInt(sc.next());
		double sum = 0;
		for(int i=1; i<=n; i++) {
			double a = sc.nextDouble();
			sum += (1/a);
		}
		System.out.println(1/sum);
	}
}