import java.util.*;
import java.io.*;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int n = sc.nextInt();
		double sum = 0;
		for(int i = 0; i < n; i++)
			sum += (1.0 / sc.nextInt());
		
		sum = 1.0 / sum;
		System.out.println(sum);
	}
} import java.util.*; util.* java import java.io.*; io.* java public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int n = sc.nextInt();
		double sum = 0;
		for(int i = 0; i < n; i++)
			sum += (1.0 / sc.nextInt());
		
		sum = 1.0 / sum;
		System.out.println(sum);
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int n = sc.nextInt();
		double sum = 0;
		for(int i = 0; i < n; i++)
			sum += (1.0 / sc.nextInt());
		
		sum = 1.0 / sum;
		System.out.println(sum);
	} main {
		Scanner sc = new Scanner(System.in);
		
		int n = sc.nextInt();
		double sum = 0;
		for(int i = 0; i < n; i++)
			sum += (1.0 / sc.nextInt());
		
		sum = 1.0 / sum;
		System.out.println(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++)
			sum += (1.0 / sc.nextInt()); int i = 0; int i = 0; i 0 i < n i n i++ i++ i sum += (1.0 / sc.nextInt()); sum += (1.0 / sc.nextInt()) sum (1.0 / sc.nextInt()) 1.0 sc.nextInt() sc.nextInt sc sum = 1.0 / sum sum 1.0 / sum 1.0 sum System.out.println(sum) System.out.println System.out System System.out sum String[] args args public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int n = sc.nextInt();
		double sum = 0;
		for(int i = 0; i < n; i++)
			sum += (1.0 / sc.nextInt());
		
		sum = 1.0 / sum;
		System.out.println(sum);
	}
} public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int n = sc.nextInt();
		double sum = 0;
		for(int i = 0; i < n; i++)
			sum += (1.0 / sc.nextInt());
		
		sum = 1.0 / sum;
		System.out.println(sum);
	}
} Main
import java.util.*;
import java.io.*;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int n = sc.nextInt();
		double sum = 0;
		for(int i = 0; i < n; i++)
			sum += (1.0 / sc.nextInt());
		
		sum = 1.0 / sum;
		System.out.println(sum);
	}
}
