import java.util.Scanner;

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

		double ans1 = 0;
		for (int i = 0; i < N; i++) {
			ans1 += 1 / (double) sc.nextInt();
		}
		sc.close();
		double ans2 = (1 / ans1);
		System.out.println(ans2);
	}
} 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 ans1 = 0;
		for (int i = 0; i < N; i++) {
			ans1 += 1 / (double) sc.nextInt();
		}
		sc.close();
		double ans2 = (1 / ans1);
		System.out.println(ans2);
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();

		double ans1 = 0;
		for (int i = 0; i < N; i++) {
			ans1 += 1 / (double) sc.nextInt();
		}
		sc.close();
		double ans2 = (1 / ans1);
		System.out.println(ans2);
	} main {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();

		double ans1 = 0;
		for (int i = 0; i < N; i++) {
			ans1 += 1 / (double) sc.nextInt();
		}
		sc.close();
		double ans2 = (1 / ans1);
		System.out.println(ans2);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc double ans1 = 0; ans1 0 for (int i = 0; i < N; i++) {
			ans1 += 1 / (double) sc.nextInt();
		} int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
			ans1 += 1 / (double) sc.nextInt();
		} {
			ans1 += 1 / (double) sc.nextInt();
		} ans1 += 1 / (double) sc.nextInt() ans1 1 / (double) sc.nextInt() 1 (double) sc.nextInt() sc.nextInt sc sc.close() sc.close sc double ans2 = (1 / ans1); ans2 (1 / ans1) 1 ans1 System.out.println(ans2) System.out.println System.out System System.out ans2 String[] args args public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();

		double ans1 = 0;
		for (int i = 0; i < N; i++) {
			ans1 += 1 / (double) sc.nextInt();
		}
		sc.close();
		double ans2 = (1 / ans1);
		System.out.println(ans2);
	}
} public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();

		double ans1 = 0;
		for (int i = 0; i < N; i++) {
			ans1 += 1 / (double) sc.nextInt();
		}
		sc.close();
		double ans2 = (1 / ans1);
		System.out.println(ans2);
	}
} 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 ans1 = 0;
		for (int i = 0; i < N; i++) {
			ans1 += 1 / (double) sc.nextInt();
		}
		sc.close();
		double ans2 = (1 / ans1);
		System.out.println(ans2);
	}
}