import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int a = s.nextInt();
		double sum = 0;
		for (int i = 0; i <a; i++) {
			int ai = s.nextInt();
			double val = 1.0/ai;
			sum=sum+val; 
		}
		System.out.println(1/sum);

	}

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

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int a = s.nextInt();
		double sum = 0;
		for (int i = 0; i <a; i++) {
			int ai = s.nextInt();
			double val = 1.0/ai;
			sum=sum+val; 
		}
		System.out.println(1/sum);

	}

} Main public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int a = s.nextInt();
		double sum = 0;
		for (int i = 0; i <a; i++) {
			int ai = s.nextInt();
			double val = 1.0/ai;
			sum=sum+val; 
		}
		System.out.println(1/sum);

	} main {
		Scanner s = new Scanner(System.in);
		int a = s.nextInt();
		double sum = 0;
		for (int i = 0; i <a; i++) {
			int ai = s.nextInt();
			double val = 1.0/ai;
			sum=sum+val; 
		}
		System.out.println(1/sum);

	} Scanner s = new Scanner(System.in); s new Scanner(System.in) int a = s.nextInt(); a s.nextInt() s.nextInt s double sum = 0; sum 0 for (int i = 0; i <a; i++) {
			int ai = s.nextInt();
			double val = 1.0/ai;
			sum=sum+val; 
		} int i = 0; int i = 0; i 0 i <a i a i++ i++ i {
			int ai = s.nextInt();
			double val = 1.0/ai;
			sum=sum+val; 
		} {
			int ai = s.nextInt();
			double val = 1.0/ai;
			sum=sum+val; 
		} int ai = s.nextInt(); ai s.nextInt() s.nextInt s double val = 1.0/ai; val 1.0/ai 1.0 ai sum=sum+val sum sum+val sum val 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 s = new Scanner(System.in);
		int a = s.nextInt();
		double sum = 0;
		for (int i = 0; i <a; i++) {
			int ai = s.nextInt();
			double val = 1.0/ai;
			sum=sum+val; 
		}
		System.out.println(1/sum);

	}

} public class Main {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int a = s.nextInt();
		double sum = 0;
		for (int i = 0; i <a; i++) {
			int ai = s.nextInt();
			double val = 1.0/ai;
			sum=sum+val; 
		}
		System.out.println(1/sum);

	}

} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int a = s.nextInt();
		double sum = 0;
		for (int i = 0; i <a; i++) {
			int ai = s.nextInt();
			double val = 1.0/ai;
			sum=sum+val; 
		}
		System.out.println(1/sum);

	}

}
