import java.util.Scanner;

public class Main {
	static int intN =0;
	static int sosuCnt =0;

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String n = sc.nextLine();
		String[] v = sc.nextLine().split(" ");

		int intN = Integer.parseInt(n);

		double result = 0;
		for (int i = 0;i < v.length;i++) {

			result = result + 1.0 / Integer.parseInt(v[i]);

		}

		System.out.println(1 / result);
	}
} import java.util.Scanner; Scanner java.util public class Main {
	static int intN =0;
	static int sosuCnt =0;

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String n = sc.nextLine();
		String[] v = sc.nextLine().split(" ");

		int intN = Integer.parseInt(n);

		double result = 0;
		for (int i = 0;i < v.length;i++) {

			result = result + 1.0 / Integer.parseInt(v[i]);

		}

		System.out.println(1 / result);
	}
} Main static int intN =0; intN 0 static int sosuCnt =0; sosuCnt 0 public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String n = sc.nextLine();
		String[] v = sc.nextLine().split(" ");

		int intN = Integer.parseInt(n);

		double result = 0;
		for (int i = 0;i < v.length;i++) {

			result = result + 1.0 / Integer.parseInt(v[i]);

		}

		System.out.println(1 / result);
	} main {
		Scanner sc = new Scanner(System.in);
		String n = sc.nextLine();
		String[] v = sc.nextLine().split(" ");

		int intN = Integer.parseInt(n);

		double result = 0;
		for (int i = 0;i < v.length;i++) {

			result = result + 1.0 / Integer.parseInt(v[i]);

		}

		System.out.println(1 / result);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) String n = sc.nextLine(); n sc.nextLine() sc.nextLine sc String[] v = sc.nextLine().split(" "); v sc.nextLine().split(" ") sc.nextLine().split sc.nextLine() sc.nextLine sc " " int intN = Integer.parseInt(n); intN Integer.parseInt(n) Integer.parseInt Integer n double result = 0; result 0 for (int i = 0;i < v.length;i++) {

			result = result + 1.0 / Integer.parseInt(v[i]);

		} int i = 0; int i = 0; i 0 i < v.length i v.length v v.length i++ i++ i {

			result = result + 1.0 / Integer.parseInt(v[i]);

		} {

			result = result + 1.0 / Integer.parseInt(v[i]);

		} result = result + 1.0 / Integer.parseInt(v[i]) result result + 1.0 / Integer.parseInt(v[i]) result 1.0 / Integer.parseInt(v[i]) 1.0 Integer.parseInt(v[i]) Integer.parseInt Integer v[i] v i System.out.println(1 / result) System.out.println System.out System System.out 1 / result 1 result String[] args args public class Main {
	static int intN =0;
	static int sosuCnt =0;

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String n = sc.nextLine();
		String[] v = sc.nextLine().split(" ");

		int intN = Integer.parseInt(n);

		double result = 0;
		for (int i = 0;i < v.length;i++) {

			result = result + 1.0 / Integer.parseInt(v[i]);

		}

		System.out.println(1 / result);
	}
} public class Main {
	static int intN =0;
	static int sosuCnt =0;

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String n = sc.nextLine();
		String[] v = sc.nextLine().split(" ");

		int intN = Integer.parseInt(n);

		double result = 0;
		for (int i = 0;i < v.length;i++) {

			result = result + 1.0 / Integer.parseInt(v[i]);

		}

		System.out.println(1 / result);
	}
} Main
import java.util.Scanner;

public class Main {
	static int intN =0;
	static int sosuCnt =0;

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String n = sc.nextLine();
		String[] v = sc.nextLine().split(" ");

		int intN = Integer.parseInt(n);

		double result = 0;
		for (int i = 0;i < v.length;i++) {

			result = result + 1.0 / Integer.parseInt(v[i]);

		}

		System.out.println(1 / result);
	}
}