import java.util.*;

public class Main {

	public static void main (String[] args)  {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double sum = 0;
		for (int i = 0; i < n; i++) {
			sum += inverse(sc.nextInt());
		}
		System.out.println(inverse(sum));
	}
	
	static double inverse(double x) {
		return 1 / x;
	}
} import java.util.*; util.* java public class Main {

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

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

	public static void main (String[] args)  {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double sum = 0;
		for (int i = 0; i < n; i++) {
			sum += inverse(sc.nextInt());
		}
		System.out.println(inverse(sum));
	}
	
	static double inverse(double x) {
		return 1 / x;
	}
} Main
import java.util.*;

public class Main {

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