import java.util.*;
public class Main {
	Scanner sc;
	int[] A;
	int N;

	public Main() {
		sc = new Scanner(System.in);
	}
	
	private void calc() {
		N = sc.nextInt();
		A = new int[N];
		for (int i = 0; i < N; i++) A[i] = sc.nextInt();
		double s = 0d;
		for (int i = 0; i < N; i++) s += 1d/A[i];
		System.out.println(1d/s);
		System.out.flush();
	}

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

} import java.util.*; util.* java public class Main {
	Scanner sc;
	int[] A;
	int N;

	public Main() {
		sc = new Scanner(System.in);
	}
	
	private void calc() {
		N = sc.nextInt();
		A = new int[N];
		for (int i = 0; i < N; i++) A[i] = sc.nextInt();
		double s = 0d;
		for (int i = 0; i < N; i++) s += 1d/A[i];
		System.out.println(1d/s);
		System.out.flush();
	}

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

} Main Scanner sc; sc int[] A; A int N; N public Main() {
		sc = new Scanner(System.in);
	} Main {
		sc = new Scanner(System.in);
	} sc = new Scanner(System.in) sc new Scanner(System.in) private void calc() {
		N = sc.nextInt();
		A = new int[N];
		for (int i = 0; i < N; i++) A[i] = sc.nextInt();
		double s = 0d;
		for (int i = 0; i < N; i++) s += 1d/A[i];
		System.out.println(1d/s);
		System.out.flush();
	} calc {
		N = sc.nextInt();
		A = new int[N];
		for (int i = 0; i < N; i++) A[i] = sc.nextInt();
		double s = 0d;
		for (int i = 0; i < N; i++) s += 1d/A[i];
		System.out.println(1d/s);
		System.out.flush();
	} N = sc.nextInt() N sc.nextInt() sc.nextInt sc 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 double s = 0d; s 0d for (int i = 0; i < N; i++) s += 1d/A[i]; int i = 0; int i = 0; i 0 i < N i N i++ i++ i s += 1d/A[i]; s += 1d/A[i] s 1d/A[i] 1d A[i] A i System.out.println(1d/s) System.out.println System.out System System.out 1d/s 1d s System.out.flush() System.out.flush System.out System System.out public static void main(String[] args) {
		new Main().calc();
	} main {
		new Main().calc();
	} new Main().calc() new Main().calc new Main() String[] args args public class Main {
	Scanner sc;
	int[] A;
	int N;

	public Main() {
		sc = new Scanner(System.in);
	}
	
	private void calc() {
		N = sc.nextInt();
		A = new int[N];
		for (int i = 0; i < N; i++) A[i] = sc.nextInt();
		double s = 0d;
		for (int i = 0; i < N; i++) s += 1d/A[i];
		System.out.println(1d/s);
		System.out.flush();
	}

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

} public class Main {
	Scanner sc;
	int[] A;
	int N;

	public Main() {
		sc = new Scanner(System.in);
	}
	
	private void calc() {
		N = sc.nextInt();
		A = new int[N];
		for (int i = 0; i < N; i++) A[i] = sc.nextInt();
		double s = 0d;
		for (int i = 0; i < N; i++) s += 1d/A[i];
		System.out.println(1d/s);
		System.out.flush();
	}

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

} Main
import java.util.*;
public class Main {
	Scanner sc;
	int[] A;
	int N;

	public Main() {
		sc = new Scanner(System.in);
	}
	
	private void calc() {
		N = sc.nextInt();
		A = new int[N];
		for (int i = 0; i < N; i++) A[i] = sc.nextInt();
		double s = 0d;
		for (int i = 0; i < N; i++) s += 1d/A[i];
		System.out.println(1d/s);
		System.out.flush();
	}

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

}