import java.util.Scanner;

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

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

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

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

		Scanner scan = new Scanner(System.in);
		
		int N = scan.nextInt();
		int[] number = new int[N];
		double sum = 0;
		
		for (int i = 0; i < N; i++) {
			number[i] = scan.nextInt();
			sum += (double)1 / number[i];
		}
	
		System.out.println(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();
		int[] number = new int[N];
		double sum = 0;
		
		for (int i = 0; i < N; i++) {
			number[i] = scan.nextInt();
			sum += (double)1 / number[i];
		}
	
		System.out.println(1 / sum);
	}
}