import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		int N = scanner.nextInt();
		
		double sum = 0.0;
		for (int i = 0; i < N; i++) {
			double a = scanner.nextDouble();
			a = 1 / a;
			sum += a;
		}
		System.out.println(1 / sum);
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		int N = scanner.nextInt();
		
		double sum = 0.0;
		for (int i = 0; i < N; i++) {
			double a = scanner.nextDouble();
			a = 1 / a;
			sum += a;
		}
		System.out.println(1 / sum);
	}
} Main public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		int N = scanner.nextInt();
		
		double sum = 0.0;
		for (int i = 0; i < N; i++) {
			double a = scanner.nextDouble();
			a = 1 / a;
			sum += a;
		}
		System.out.println(1 / sum);
	} main {
		Scanner scanner = new Scanner(System.in);
		int N = scanner.nextInt();
		
		double sum = 0.0;
		for (int i = 0; i < N; i++) {
			double a = scanner.nextDouble();
			a = 1 / a;
			sum += a;
		}
		System.out.println(1 / sum);
	} Scanner scanner = new Scanner(System.in); scanner new Scanner(System.in) int N = scanner.nextInt(); N scanner.nextInt() scanner.nextInt scanner double sum = 0.0; sum 0.0 for (int i = 0; i < N; i++) {
			double a = scanner.nextDouble();
			a = 1 / a;
			sum += a;
		} int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
			double a = scanner.nextDouble();
			a = 1 / a;
			sum += a;
		} {
			double a = scanner.nextDouble();
			a = 1 / a;
			sum += a;
		} double a = scanner.nextDouble(); a scanner.nextDouble() scanner.nextDouble scanner a = 1 / a a 1 / a 1 a sum += a sum a 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 scanner = new Scanner(System.in);
		int N = scanner.nextInt();
		
		double sum = 0.0;
		for (int i = 0; i < N; i++) {
			double a = scanner.nextDouble();
			a = 1 / a;
			sum += a;
		}
		System.out.println(1 / sum);
	}
} public class Main {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		int N = scanner.nextInt();
		
		double sum = 0.0;
		for (int i = 0; i < N; i++) {
			double a = scanner.nextDouble();
			a = 1 / a;
			sum += a;
		}
		System.out.println(1 / sum);
	}
} Main
import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		int N = scanner.nextInt();
		
		double sum = 0.0;
		for (int i = 0; i < N; i++) {
			double a = scanner.nextDouble();
			a = 1 / a;
			sum += a;
		}
		System.out.println(1 / sum);
	}
}
