import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
		         int line []= new int [N];
		         double tos=0.0;

				 for(int i=0;i<N;i++) {
				 line[i]  =sc.nextInt();

				 tos+=(double)1/line[i];

				 }

		System.out.println(1/tos);



	}



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

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
		         int line []= new int [N];
		         double tos=0.0;

				 for(int i=0;i<N;i++) {
				 line[i]  =sc.nextInt();

				 tos+=(double)1/line[i];

				 }

		System.out.println(1/tos);



	}



} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
		         int line []= new int [N];
		         double tos=0.0;

				 for(int i=0;i<N;i++) {
				 line[i]  =sc.nextInt();

				 tos+=(double)1/line[i];

				 }

		System.out.println(1/tos);



	} main {
		Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
		         int line []= new int [N];
		         double tos=0.0;

				 for(int i=0;i<N;i++) {
				 line[i]  =sc.nextInt();

				 tos+=(double)1/line[i];

				 }

		System.out.println(1/tos);



	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc int line []= new int [N]; line new int [N] N double tos=0.0; tos 0.0 for(int i=0;i<N;i++) {
				 line[i]  =sc.nextInt();

				 tos+=(double)1/line[i];

				 } int i=0; int i=0; i 0 i<N i N i++ i++ i {
				 line[i]  =sc.nextInt();

				 tos+=(double)1/line[i];

				 } {
				 line[i]  =sc.nextInt();

				 tos+=(double)1/line[i];

				 } line[i]  =sc.nextInt() line[i] line i sc.nextInt() sc.nextInt sc tos+=(double)1/line[i] tos (double)1/line[i] (double)1 line[i] line i System.out.println(1/tos) System.out.println System.out System System.out 1/tos 1 tos String[] args args public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
		         int line []= new int [N];
		         double tos=0.0;

				 for(int i=0;i<N;i++) {
				 line[i]  =sc.nextInt();

				 tos+=(double)1/line[i];

				 }

		System.out.println(1/tos);



	}



} public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
		         int line []= new int [N];
		         double tos=0.0;

				 for(int i=0;i<N;i++) {
				 line[i]  =sc.nextInt();

				 tos+=(double)1/line[i];

				 }

		System.out.println(1/tos);



	}



} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
		         int line []= new int [N];
		         double tos=0.0;

				 for(int i=0;i<N;i++) {
				 line[i]  =sc.nextInt();

				 tos+=(double)1/line[i];

				 }

		System.out.println(1/tos);



	}



}