import java.util.*;

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

class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a =    sc.nextInt();
		double num = 0;
		for(int i = 0; i < a;i++) {
			int b =    sc.nextInt();
			num += 1.0 /b;
		}
		 num = 1.0 / num;  
       System.out.println(num);
	}
}