import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();

		double[] a = new double[n];
		double result = 0;
		for (int i = 0; i < n; i++) {
			a[i] = sc.nextDouble();
			result += 1 / a[i];
		}
		System.out.println(1 / result);
		sc.close();
	}

} 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[] a = new double[n];
		double result = 0;
		for (int i = 0; i < n; i++) {
			a[i] = sc.nextDouble();
			result += 1 / a[i];
		}
		System.out.println(1 / result);
		sc.close();
	}

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

		int n = sc.nextInt();

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

		int n = sc.nextInt();

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

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();

		double[] a = new double[n];
		double result = 0;
		for (int i = 0; i < n; i++) {
			a[i] = sc.nextDouble();
			result += 1 / a[i];
		}
		System.out.println(1 / result);
		sc.close();
	}

} public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();

		double[] a = new double[n];
		double result = 0;
		for (int i = 0; i < n; i++) {
			a[i] = sc.nextDouble();
			result += 1 / a[i];
		}
		System.out.println(1 / result);
		sc.close();
	}

} 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[] a = new double[n];
		double result = 0;
		for (int i = 0; i < n; i++) {
			a[i] = sc.nextDouble();
			result += 1 / a[i];
		}
		System.out.println(1 / result);
		sc.close();
	}

}
