import java.util.*;
 
public class Main{
	public static void main(String[] args){
    	Scanner scan = new Scanner(System.in);
      	int n = scan.nextInt();
      	int[] a = new int[n];
      	for (int i = 0; i < n; i++){
        	a[i] = scan.nextInt();
        }
      	double bunbo = 0.0;
      	for(int i = 0; i < n; i++){
        bunbo += (double)1 / a[i];
        }
      	System.out.print(1 / bunbo);
    }
} import java.util.*; util.* java public class Main{
	public static void main(String[] args){
    	Scanner scan = new Scanner(System.in);
      	int n = scan.nextInt();
      	int[] a = new int[n];
      	for (int i = 0; i < n; i++){
        	a[i] = scan.nextInt();
        }
      	double bunbo = 0.0;
      	for(int i = 0; i < n; i++){
        bunbo += (double)1 / a[i];
        }
      	System.out.print(1 / bunbo);
    }
} Main public static void main(String[] args){
    	Scanner scan = new Scanner(System.in);
      	int n = scan.nextInt();
      	int[] a = new int[n];
      	for (int i = 0; i < n; i++){
        	a[i] = scan.nextInt();
        }
      	double bunbo = 0.0;
      	for(int i = 0; i < n; i++){
        bunbo += (double)1 / a[i];
        }
      	System.out.print(1 / bunbo);
    } main {
    	Scanner scan = new Scanner(System.in);
      	int n = scan.nextInt();
      	int[] a = new int[n];
      	for (int i = 0; i < n; i++){
        	a[i] = scan.nextInt();
        }
      	double bunbo = 0.0;
      	for(int i = 0; i < n; i++){
        bunbo += (double)1 / a[i];
        }
      	System.out.print(1 / bunbo);
    } Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int n = scan.nextInt(); n scan.nextInt() scan.nextInt scan int[] a = new int[n]; a new int[n] n for (int i = 0; i < n; i++){
        	a[i] = scan.nextInt();
        } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
        	a[i] = scan.nextInt();
        } {
        	a[i] = scan.nextInt();
        } a[i] = scan.nextInt() a[i] a i scan.nextInt() scan.nextInt scan double bunbo = 0.0; bunbo 0.0 for(int i = 0; i < n; i++){
        bunbo += (double)1 / a[i];
        } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
        bunbo += (double)1 / a[i];
        } {
        bunbo += (double)1 / a[i];
        } bunbo += (double)1 / a[i] bunbo (double)1 / a[i] (double)1 a[i] a i System.out.print(1 / bunbo) System.out.print System.out System System.out 1 / bunbo 1 bunbo String[] args args public class Main{
	public static void main(String[] args){
    	Scanner scan = new Scanner(System.in);
      	int n = scan.nextInt();
      	int[] a = new int[n];
      	for (int i = 0; i < n; i++){
        	a[i] = scan.nextInt();
        }
      	double bunbo = 0.0;
      	for(int i = 0; i < n; i++){
        bunbo += (double)1 / a[i];
        }
      	System.out.print(1 / bunbo);
    }
} public class Main{
	public static void main(String[] args){
    	Scanner scan = new Scanner(System.in);
      	int n = scan.nextInt();
      	int[] a = new int[n];
      	for (int i = 0; i < n; i++){
        	a[i] = scan.nextInt();
        }
      	double bunbo = 0.0;
      	for(int i = 0; i < n; i++){
        bunbo += (double)1 / a[i];
        }
      	System.out.print(1 / bunbo);
    }
} Main
import java.util.*;
 
public class Main{
	public static void main(String[] args){
    	Scanner scan = new Scanner(System.in);
      	int n = scan.nextInt();
      	int[] a = new int[n];
      	for (int i = 0; i < n; i++){
        	a[i] = scan.nextInt();
        }
      	double bunbo = 0.0;
      	for(int i = 0; i < n; i++){
        bunbo += (double)1 / a[i];
        }
      	System.out.print(1 / bunbo);
    }
}