
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();

		double sum = 0;
		for(int i = 0; i < N; i++) {
			double num = sc.nextDouble();
			sum += 1 / num;
		}

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

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

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();

		double sum = 0;
		for(int i = 0; i < N; i++) {
			double num = sc.nextDouble();
			sum += 1 / num;
		}

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

} Main public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();

		double sum = 0;
		for(int i = 0; i < N; i++) {
			double num = sc.nextDouble();
			sum += 1 / num;
		}

		System.out.println(1 / sum);
	} main {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();

		double sum = 0;
		for(int i = 0; i < N; i++) {
			double num = sc.nextDouble();
			sum += 1 / num;
		}

		System.out.println(1 / sum);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc double sum = 0; sum 0 for(int i = 0; i < N; i++) {
			double num = sc.nextDouble();
			sum += 1 / num;
		} int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
			double num = sc.nextDouble();
			sum += 1 / num;
		} {
			double num = sc.nextDouble();
			sum += 1 / num;
		} double num = sc.nextDouble(); num sc.nextDouble() sc.nextDouble sc sum += 1 / num sum 1 / num 1 num 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) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();

		double sum = 0;
		for(int i = 0; i < N; i++) {
			double num = sc.nextDouble();
			sum += 1 / num;
		}

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

} public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();

		double sum = 0;
		for(int i = 0; i < N; i++) {
			double num = sc.nextDouble();
			sum += 1 / num;
		}

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

} Main

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();

		double sum = 0;
		for(int i = 0; i < N; i++) {
			double num = sc.nextDouble();
			sum += 1 / num;
		}

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

}
