/*package whatever //do not write package name here */

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

class Main {
	public static void main (String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();int mul=1;double sum=0;
		for(int i=0;i<N;i++){
		    double a=sc.nextDouble();
		   // mul=mul*a;
		    sum=sum+1/a;
		}
		//double b=(double)sum/(double)mul;
		System.out.println(1/sum);

	}
} import java.io.*; io.* java import java.util.Scanner; Scanner java.util class Main {
	public static void main (String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();int mul=1;double sum=0;
		for(int i=0;i<N;i++){
		    double a=sc.nextDouble();
		   // mul=mul*a;
		    sum=sum+1/a;
		}
		//double b=(double)sum/(double)mul;
		System.out.println(1/sum);

	}
} Main public static void main (String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();int mul=1;double sum=0;
		for(int i=0;i<N;i++){
		    double a=sc.nextDouble();
		   // mul=mul*a;
		    sum=sum+1/a;
		}
		//double b=(double)sum/(double)mul;
		System.out.println(1/sum);

	} main {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();int mul=1;double sum=0;
		for(int i=0;i<N;i++){
		    double a=sc.nextDouble();
		   // mul=mul*a;
		    sum=sum+1/a;
		}
		//double b=(double)sum/(double)mul;
		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 int mul=1; mul 1 double sum=0; sum 0 for(int i=0;i<N;i++){
		    double a=sc.nextDouble();
		   // mul=mul*a;
		    sum=sum+1/a;
		} int i=0; int i=0; i 0 i<N i N i++ i++ i {
		    double a=sc.nextDouble();
		   // mul=mul*a;
		    sum=sum+1/a;
		} {
		    double a=sc.nextDouble();
		   // mul=mul*a;
		    sum=sum+1/a;
		} double a=sc.nextDouble(); a sc.nextDouble() sc.nextDouble sc sum=sum+1/a sum sum+1/a sum 1/a 1 a System.out.println(1/sum) System.out.println System.out System System.out 1/sum 1 sum String[] args args
/*package whatever //do not write package name here */

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

class Main {
	public static void main (String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();int mul=1;double sum=0;
		for(int i=0;i<N;i++){
		    double a=sc.nextDouble();
		   // mul=mul*a;
		    sum=sum+1/a;
		}
		//double b=(double)sum/(double)mul;
		System.out.println(1/sum);

	}
}