import java.util.Scanner;

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

		Scanner scan = new Scanner(System.in);
		int N = scan.nextInt();
		int[] A = new int[N];
		double total = 0.0;

		for (int i = 0; i < A.length; i++) {
			A[i] = scan.nextInt();

		}


		for(int i = 0; i<N; i++) {
			total += (double)1/A[i];
		}

		System.out.println(1/total);

	}




} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);
		int N = scan.nextInt();
		int[] A = new int[N];
		double total = 0.0;

		for (int i = 0; i < A.length; i++) {
			A[i] = scan.nextInt();

		}


		for(int i = 0; i<N; i++) {
			total += (double)1/A[i];
		}

		System.out.println(1/total);

	}




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

		Scanner scan = new Scanner(System.in);
		int N = scan.nextInt();
		int[] A = new int[N];
		double total = 0.0;

		for (int i = 0; i < A.length; i++) {
			A[i] = scan.nextInt();

		}


		for(int i = 0; i<N; i++) {
			total += (double)1/A[i];
		}

		System.out.println(1/total);

	} main {

		Scanner scan = new Scanner(System.in);
		int N = scan.nextInt();
		int[] A = new int[N];
		double total = 0.0;

		for (int i = 0; i < A.length; i++) {
			A[i] = scan.nextInt();

		}


		for(int i = 0; i<N; i++) {
			total += (double)1/A[i];
		}

		System.out.println(1/total);

	} Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int N = scan.nextInt(); N scan.nextInt() scan.nextInt scan int[] A = new int[N]; A new int[N] N double total = 0.0; total 0.0 for (int i = 0; i < A.length; i++) {
			A[i] = scan.nextInt();

		} int i = 0; int i = 0; i 0 i < A.length i A.length A A.length i++ i++ i {
			A[i] = scan.nextInt();

		} {
			A[i] = scan.nextInt();

		} A[i] = scan.nextInt() A[i] A i scan.nextInt() scan.nextInt scan for(int i = 0; i<N; i++) {
			total += (double)1/A[i];
		} int i = 0; int i = 0; i 0 i<N i N i++ i++ i {
			total += (double)1/A[i];
		} {
			total += (double)1/A[i];
		} total += (double)1/A[i] total (double)1/A[i] (double)1 A[i] A i System.out.println(1/total) System.out.println System.out System System.out 1/total 1 total String[] args args public class Main {
	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);
		int N = scan.nextInt();
		int[] A = new int[N];
		double total = 0.0;

		for (int i = 0; i < A.length; i++) {
			A[i] = scan.nextInt();

		}


		for(int i = 0; i<N; i++) {
			total += (double)1/A[i];
		}

		System.out.println(1/total);

	}




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

		Scanner scan = new Scanner(System.in);
		int N = scan.nextInt();
		int[] A = new int[N];
		double total = 0.0;

		for (int i = 0; i < A.length; i++) {
			A[i] = scan.nextInt();

		}


		for(int i = 0; i<N; i++) {
			total += (double)1/A[i];
		}

		System.out.println(1/total);

	}




} Main
import java.util.Scanner;

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

		Scanner scan = new Scanner(System.in);
		int N = scan.nextInt();
		int[] A = new int[N];
		double total = 0.0;

		for (int i = 0; i < A.length; i++) {
			A[i] = scan.nextInt();

		}


		for(int i = 0; i<N; i++) {
			total += (double)1/A[i];
		}

		System.out.println(1/total);

	}




}
