import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int n= scan.nextInt();
		double sum=0;
		for(int i=0;i<n;i++) {
			sum+=(double)1/scan.nextInt();
		}

		scan.close();
		System.out.println((double)1/sum);
	}

} import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int n= scan.nextInt();
		double sum=0;
		for(int i=0;i<n;i++) {
			sum+=(double)1/scan.nextInt();
		}

		scan.close();
		System.out.println((double)1/sum);
	}

} Main public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int n= scan.nextInt();
		double sum=0;
		for(int i=0;i<n;i++) {
			sum+=(double)1/scan.nextInt();
		}

		scan.close();
		System.out.println((double)1/sum);
	} main {
		Scanner scan = new Scanner(System.in);
		int n= scan.nextInt();
		double sum=0;
		for(int i=0;i<n;i++) {
			sum+=(double)1/scan.nextInt();
		}

		scan.close();
		System.out.println((double)1/sum);
	} Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int n= scan.nextInt(); n scan.nextInt() scan.nextInt scan double sum=0; sum 0 for(int i=0;i<n;i++) {
			sum+=(double)1/scan.nextInt();
		} int i=0; int i=0; i 0 i<n i n i++ i++ i {
			sum+=(double)1/scan.nextInt();
		} {
			sum+=(double)1/scan.nextInt();
		} sum+=(double)1/scan.nextInt() sum (double)1/scan.nextInt() (double)1 scan.nextInt() scan.nextInt scan scan.close() scan.close scan System.out.println((double)1/sum) System.out.println System.out System System.out (double)1/sum (double)1 sum String[] args args public class Main {

	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int n= scan.nextInt();
		double sum=0;
		for(int i=0;i<n;i++) {
			sum+=(double)1/scan.nextInt();
		}

		scan.close();
		System.out.println((double)1/sum);
	}

} public class Main {

	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int n= scan.nextInt();
		double sum=0;
		for(int i=0;i<n;i++) {
			sum+=(double)1/scan.nextInt();
		}

		scan.close();
		System.out.println((double)1/sum);
	}

} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int n= scan.nextInt();
		double sum=0;
		for(int i=0;i<n;i++) {
			sum+=(double)1/scan.nextInt();
		}

		scan.close();
		System.out.println((double)1/sum);
	}

}
