import java.util.*;
import java.lang.*;
class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
      	double num = 0.0;
      	int child = 1;
     	int mom = 1;
     	for(int i = 0; i < a; i++){
          num += 1.0/sc.nextInt();
        }
        System.out.println(1.0/num);
    }
} import java.util.*; util.* java import java.lang.*; lang.* java class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
      	double num = 0.0;
      	int child = 1;
     	int mom = 1;
     	for(int i = 0; i < a; i++){
          num += 1.0/sc.nextInt();
        }
        System.out.println(1.0/num);
    }
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
      	double num = 0.0;
      	int child = 1;
     	int mom = 1;
     	for(int i = 0; i < a; i++){
          num += 1.0/sc.nextInt();
        }
        System.out.println(1.0/num);
    } main {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
      	double num = 0.0;
      	int child = 1;
     	int mom = 1;
     	for(int i = 0; i < a; i++){
          num += 1.0/sc.nextInt();
        }
        System.out.println(1.0/num);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int a = sc.nextInt(); a sc.nextInt() sc.nextInt sc double num = 0.0; num 0.0 int child = 1; child 1 int mom = 1; mom 1 for(int i = 0; i < a; i++){
          num += 1.0/sc.nextInt();
        } int i = 0; int i = 0; i 0 i < a i a i++ i++ i {
          num += 1.0/sc.nextInt();
        } {
          num += 1.0/sc.nextInt();
        } num += 1.0/sc.nextInt() num 1.0/sc.nextInt() 1.0 sc.nextInt() sc.nextInt sc System.out.println(1.0/num) System.out.println System.out System System.out 1.0/num 1.0 num String[] args args
import java.util.*;
import java.lang.*;
class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
      	double num = 0.0;
      	int child = 1;
     	int mom = 1;
     	for(int i = 0; i < a; i++){
          num += 1.0/sc.nextInt();
        }
        System.out.println(1.0/num);
    }
}