import java.util.Scanner;

class Main {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] a = new int[n];
		for (int i = 0; i < n; i++)
			a[i] = sc.nextInt();
		sc.close();
		double m = 0;
		double l;

		for (int i = 0; i < n; i++)
			m = m + (double) 1 / a[i];

		l = 1 / m;

		System.out.println(l);

	}
} import java.util.Scanner; Scanner java.util class Main {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] a = new int[n];
		for (int i = 0; i < n; i++)
			a[i] = sc.nextInt();
		sc.close();
		double m = 0;
		double l;

		for (int i = 0; i < n; i++)
			m = m + (double) 1 / a[i];

		l = 1 / m;

		System.out.println(l);

	}
} Main public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] a = new int[n];
		for (int i = 0; i < n; i++)
			a[i] = sc.nextInt();
		sc.close();
		double m = 0;
		double l;

		for (int i = 0; i < n; i++)
			m = m + (double) 1 / a[i];

		l = 1 / m;

		System.out.println(l);

	} main {

		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] a = new int[n];
		for (int i = 0; i < n; i++)
			a[i] = sc.nextInt();
		sc.close();
		double m = 0;
		double l;

		for (int i = 0; i < n; i++)
			m = m + (double) 1 / a[i];

		l = 1 / m;

		System.out.println(l);

	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int[] a = new int[n]; a new int[n] n for (int i = 0; i < n; i++)
			a[i] = sc.nextInt(); int i = 0; int i = 0; i 0 i < n i n i++ i++ i a[i] = sc.nextInt(); a[i] = sc.nextInt() a[i] a i sc.nextInt() sc.nextInt sc sc.close() sc.close sc double m = 0; m 0 double l; l for (int i = 0; i < n; i++)
			m = m + (double) 1 / a[i]; int i = 0; int i = 0; i 0 i < n i n i++ i++ i m = m + (double) 1 / a[i]; m = m + (double) 1 / a[i] m m + (double) 1 / a[i] m (double) 1 / a[i] (double) 1 a[i] a i l = 1 / m l 1 / m 1 m System.out.println(l) System.out.println System.out System System.out l String[] args args
import java.util.Scanner;

class Main {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] a = new int[n];
		for (int i = 0; i < n; i++)
			a[i] = sc.nextInt();
		sc.close();
		double m = 0;
		double l;

		for (int i = 0; i < n; i++)
			m = m + (double) 1 / a[i];

		l = 1 / m;

		System.out.println(l);

	}
}