import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int n = scan.nextInt();
		int[] a = new int[n];

		int i;
		double sum = 0;
		for(i = 0; i < n; i++) {
			sum += 1 / scan.nextDouble();
		}
		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[] a = new int[n];

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

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

		int i;
		double sum = 0;
		for(i = 0; i < n; i++) {
			sum += 1 / scan.nextDouble();
		}
		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[] a = new int[n]; a new int[n] n int i; i double sum = 0; sum 0 for(i = 0; i < n; i++) {
			sum += 1 / scan.nextDouble();
		} i = 0; i = 0 i 0 i < n i n i++ i++ i {
			sum += 1 / scan.nextDouble();
		} {
			sum += 1 / scan.nextDouble();
		} sum += 1 / scan.nextDouble() sum 1 / scan.nextDouble() 1 scan.nextDouble() scan.nextDouble scan 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[] a = new int[n];

		int i;
		double sum = 0;
		for(i = 0; i < n; i++) {
			sum += 1 / scan.nextDouble();
		}
		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[] a = new int[n];

		int i;
		double sum = 0;
		for(i = 0; i < n; i++) {
			sum += 1 / scan.nextDouble();
		}
		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[] a = new int[n];

		int i;
		double sum = 0;
		for(i = 0; i < n; i++) {
			sum += 1 / scan.nextDouble();
		}
		System.out.println(1 / sum);
	}
}