import java.util.Arrays;
import java.util.Scanner;

class Main {
	public static void main(String[] args) {
		new Main().run();
	}

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

	void tr(Object... objects) {
		System.out.println(Arrays.deepToString(objects));
	}
} import java.util.Arrays; Arrays java.util import java.util.Scanner; Scanner java.util class Main {
	public static void main(String[] args) {
		new Main().run();
	}

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

	void tr(Object... objects) {
		System.out.println(Arrays.deepToString(objects));
	}
} Main public static void main(String[] args) {
		new Main().run();
	} main {
		new Main().run();
	} new Main().run() new Main().run new Main() String[] args args void run() {
		Scanner sc = new Scanner(System.in);
		double ans = 0;
		int n = sc.nextInt();
		double[] a = new double[n];
		for (int i = 0; i < n; ++i) {
			a[i] = sc.nextDouble();
			ans += 1 / a[i];
		}
		ans = 1 / ans;
		System.out.println(ans);
	} run {
		Scanner sc = new Scanner(System.in);
		double ans = 0;
		int n = sc.nextInt();
		double[] a = new double[n];
		for (int i = 0; i < n; ++i) {
			a[i] = sc.nextDouble();
			ans += 1 / a[i];
		}
		ans = 1 / ans;
		System.out.println(ans);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) double ans = 0; ans 0 int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc double[] a = new double[n]; a new double[n] n for (int i = 0; i < n; ++i) {
			a[i] = sc.nextDouble();
			ans += 1 / a[i];
		} int i = 0; int i = 0; i 0 i < n i n ++i ++i i {
			a[i] = sc.nextDouble();
			ans += 1 / a[i];
		} {
			a[i] = sc.nextDouble();
			ans += 1 / a[i];
		} a[i] = sc.nextDouble() a[i] a i sc.nextDouble() sc.nextDouble sc ans += 1 / a[i] ans 1 / a[i] 1 a[i] a i ans = 1 / ans ans 1 / ans 1 ans System.out.println(ans) System.out.println System.out System System.out ans void tr(Object... objects) {
		System.out.println(Arrays.deepToString(objects));
	} tr {
		System.out.println(Arrays.deepToString(objects));
	} System.out.println(Arrays.deepToString(objects)) System.out.println System.out System System.out Arrays.deepToString(objects) Arrays.deepToString Arrays objects Object... objects objects
import java.util.Arrays;
import java.util.Scanner;

class Main {
	public static void main(String[] args) {
		new Main().run();
	}

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

	void tr(Object... objects) {
		System.out.println(Arrays.deepToString(objects));
	}
}