import java.util.*;

class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int num = sc.nextInt();
		int[] nums = new int[num];
		int sum = 0;
		double divisor = 0, dividend = 1;
		for(int i = 0; i < num; i++){
			nums[i] = sc.nextInt();
			dividend *= nums[i];
		}
		for(int i : nums){
			divisor += dividend / i;
		}
		double ans = dividend / divisor;

		System.out.println(ans);
	}
}

 import java.util.*; util.* java class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int num = sc.nextInt();
		int[] nums = new int[num];
		int sum = 0;
		double divisor = 0, dividend = 1;
		for(int i = 0; i < num; i++){
			nums[i] = sc.nextInt();
			dividend *= nums[i];
		}
		for(int i : nums){
			divisor += dividend / i;
		}
		double ans = dividend / divisor;

		System.out.println(ans);
	}
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int num = sc.nextInt();
		int[] nums = new int[num];
		int sum = 0;
		double divisor = 0, dividend = 1;
		for(int i = 0; i < num; i++){
			nums[i] = sc.nextInt();
			dividend *= nums[i];
		}
		for(int i : nums){
			divisor += dividend / i;
		}
		double ans = dividend / divisor;

		System.out.println(ans);
	} main {
		Scanner sc = new Scanner(System.in);
		int num = sc.nextInt();
		int[] nums = new int[num];
		int sum = 0;
		double divisor = 0, dividend = 1;
		for(int i = 0; i < num; i++){
			nums[i] = sc.nextInt();
			dividend *= nums[i];
		}
		for(int i : nums){
			divisor += dividend / i;
		}
		double ans = dividend / divisor;

		System.out.println(ans);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int num = sc.nextInt(); num sc.nextInt() sc.nextInt sc int[] nums = new int[num]; nums new int[num] num int sum = 0; sum 0 double divisor = 0 divisor 0 dividend = 1; dividend 1 for(int i = 0; i < num; i++){
			nums[i] = sc.nextInt();
			dividend *= nums[i];
		} int i = 0; int i = 0; i 0 i < num i num i++ i++ i {
			nums[i] = sc.nextInt();
			dividend *= nums[i];
		} {
			nums[i] = sc.nextInt();
			dividend *= nums[i];
		} nums[i] = sc.nextInt() nums[i] nums i sc.nextInt() sc.nextInt sc dividend *= nums[i] dividend nums[i] nums i for(int i : nums){
			divisor += dividend / i;
		} int i nums {
			divisor += dividend / i;
		} {
			divisor += dividend / i;
		} divisor += dividend / i divisor dividend / i dividend i double ans = dividend / divisor; ans dividend / divisor dividend divisor System.out.println(ans) System.out.println System.out System System.out ans String[] args args
import java.util.*;

class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int num = sc.nextInt();
		int[] nums = new int[num];
		int sum = 0;
		double divisor = 0, dividend = 1;
		for(int i = 0; i < num; i++){
			nums[i] = sc.nextInt();
			dividend *= nums[i];
		}
		for(int i : nums){
			divisor += dividend / i;
		}
		double ans = dividend / divisor;

		System.out.println(ans);
	}
}


