import java.util.*;
//import java.util.stream.Collectors;

public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		
		int n = Integer.parseInt(sc.next());
		double sum = 0;
		for(int i=0; i<n; i++){
			sum += 1.0 / Integer.parseInt(sc.next());
		}
		
		System.out.println(1 / sum);
	}
} import java.util.*; util.* java public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		
		int n = Integer.parseInt(sc.next());
		double sum = 0;
		for(int i=0; i<n; i++){
			sum += 1.0 / Integer.parseInt(sc.next());
		}
		
		System.out.println(1 / sum);
	}
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		
		int n = Integer.parseInt(sc.next());
		double sum = 0;
		for(int i=0; i<n; i++){
			sum += 1.0 / Integer.parseInt(sc.next());
		}
		
		System.out.println(1 / sum);
	} main {
		Scanner sc = new Scanner(System.in);
		
		int n = Integer.parseInt(sc.next());
		double sum = 0;
		for(int i=0; i<n; i++){
			sum += 1.0 / Integer.parseInt(sc.next());
		}
		
		System.out.println(1 / sum);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = Integer.parseInt(sc.next()); n Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc double sum = 0; sum 0 for(int i=0; i<n; i++){
			sum += 1.0 / Integer.parseInt(sc.next());
		} int i=0; int i=0; i 0 i<n i n i++ i++ i {
			sum += 1.0 / Integer.parseInt(sc.next());
		} {
			sum += 1.0 / Integer.parseInt(sc.next());
		} sum += 1.0 / Integer.parseInt(sc.next()) sum 1.0 / Integer.parseInt(sc.next()) 1.0 Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc 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){
		Scanner sc = new Scanner(System.in);
		
		int n = Integer.parseInt(sc.next());
		double sum = 0;
		for(int i=0; i<n; i++){
			sum += 1.0 / Integer.parseInt(sc.next());
		}
		
		System.out.println(1 / sum);
	}
} public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		
		int n = Integer.parseInt(sc.next());
		double sum = 0;
		for(int i=0; i<n; i++){
			sum += 1.0 / Integer.parseInt(sc.next());
		}
		
		System.out.println(1 / sum);
	}
} Main
import java.util.*;
//import java.util.stream.Collectors;

public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		
		int n = Integer.parseInt(sc.next());
		double sum = 0;
		for(int i=0; i<n; i++){
			sum += 1.0 / Integer.parseInt(sc.next());
		}
		
		System.out.println(1 / sum);
	}
}
