import java.util.*;
import java.io.*;
class Main {
	public static void main(String[] args){
    
    		Scanner sc = new Scanner(System.in);
      		int n = sc.nextInt();
      		float sum = 0;
      		for(int i=0;i<n;i++){
            
            	float x = sc.nextFloat();
            	
            	sum += (1/x);
            }
    		System.out.println(1/sum);
    
    }



} import java.util.*; util.* java import java.io.*; io.* java class Main {
	public static void main(String[] args){
    
    		Scanner sc = new Scanner(System.in);
      		int n = sc.nextInt();
      		float sum = 0;
      		for(int i=0;i<n;i++){
            
            	float x = sc.nextFloat();
            	
            	sum += (1/x);
            }
    		System.out.println(1/sum);
    
    }



} Main public static void main(String[] args){
    
    		Scanner sc = new Scanner(System.in);
      		int n = sc.nextInt();
      		float sum = 0;
      		for(int i=0;i<n;i++){
            
            	float x = sc.nextFloat();
            	
            	sum += (1/x);
            }
    		System.out.println(1/sum);
    
    } main {
    
    		Scanner sc = new Scanner(System.in);
      		int n = sc.nextInt();
      		float sum = 0;
      		for(int i=0;i<n;i++){
            
            	float x = sc.nextFloat();
            	
            	sum += (1/x);
            }
    		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 float sum = 0; sum 0 for(int i=0;i<n;i++){
            
            	float x = sc.nextFloat();
            	
            	sum += (1/x);
            } int i=0; int i=0; i 0 i<n i n i++ i++ i {
            
            	float x = sc.nextFloat();
            	
            	sum += (1/x);
            } {
            
            	float x = sc.nextFloat();
            	
            	sum += (1/x);
            } float x = sc.nextFloat(); x sc.nextFloat() sc.nextFloat sc sum += (1/x) sum (1/x) 1 x System.out.println(1/sum) System.out.println System.out System System.out 1/sum 1 sum String[] args args
import java.util.*;
import java.io.*;
class Main {
	public static void main(String[] args){
    
    		Scanner sc = new Scanner(System.in);
      		int n = sc.nextInt();
      		float sum = 0;
      		for(int i=0;i<n;i++){
            
            	float x = sc.nextFloat();
            	
            	sum += (1/x);
            }
    		System.out.println(1/sum);
    
    }



}