import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double d =0.0;
		for (int i=0;i<n;i++) {
			int temp = sc.nextInt();
			d+=((double)1/(double)temp);
		}
		System.out.println((double)1/d);

	}
} 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();
		double d =0.0;
		for (int i=0;i<n;i++) {
			int temp = sc.nextInt();
			d+=((double)1/(double)temp);
		}
		System.out.println((double)1/d);

	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double d =0.0;
		for (int i=0;i<n;i++) {
			int temp = sc.nextInt();
			d+=((double)1/(double)temp);
		}
		System.out.println((double)1/d);

	} main {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double d =0.0;
		for (int i=0;i<n;i++) {
			int temp = sc.nextInt();
			d+=((double)1/(double)temp);
		}
		System.out.println((double)1/d);

	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc double d =0.0; d 0.0 for (int i=0;i<n;i++) {
			int temp = sc.nextInt();
			d+=((double)1/(double)temp);
		} int i=0; int i=0; i 0 i<n i n i++ i++ i {
			int temp = sc.nextInt();
			d+=((double)1/(double)temp);
		} {
			int temp = sc.nextInt();
			d+=((double)1/(double)temp);
		} int temp = sc.nextInt(); temp sc.nextInt() sc.nextInt sc d+=((double)1/(double)temp) d ((double)1/(double)temp) (double)1 (double)temp System.out.println((double)1/d) System.out.println System.out System System.out (double)1/d (double)1 d String[] args args public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double d =0.0;
		for (int i=0;i<n;i++) {
			int temp = sc.nextInt();
			d+=((double)1/(double)temp);
		}
		System.out.println((double)1/d);

	}
} public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double d =0.0;
		for (int i=0;i<n;i++) {
			int temp = sc.nextInt();
			d+=((double)1/(double)temp);
		}
		System.out.println((double)1/d);

	}
} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double d =0.0;
		for (int i=0;i<n;i++) {
			int temp = sc.nextInt();
			d+=((double)1/(double)temp);
		}
		System.out.println((double)1/d);

	}
}