import java.util.*;
public class Main {

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

	public static void main(String[] ble){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double inv = 0;
		for(int i = 0; i < n; i++){
			double a = sc.nextDouble();
			inv += 1/a;
		}
		System.out.println(1/inv);
	}
} Main public static void main(String[] ble){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double inv = 0;
		for(int i = 0; i < n; i++){
			double a = sc.nextDouble();
			inv += 1/a;
		}
		System.out.println(1/inv);
	} main {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double inv = 0;
		for(int i = 0; i < n; i++){
			double a = sc.nextDouble();
			inv += 1/a;
		}
		System.out.println(1/inv);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc double inv = 0; inv 0 for(int i = 0; i < n; i++){
			double a = sc.nextDouble();
			inv += 1/a;
		} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
			double a = sc.nextDouble();
			inv += 1/a;
		} {
			double a = sc.nextDouble();
			inv += 1/a;
		} double a = sc.nextDouble(); a sc.nextDouble() sc.nextDouble sc inv += 1/a inv 1/a 1 a System.out.println(1/inv) System.out.println System.out System System.out 1/inv 1 inv String[] ble ble public class Main {

	public static void main(String[] ble){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double inv = 0;
		for(int i = 0; i < n; i++){
			double a = sc.nextDouble();
			inv += 1/a;
		}
		System.out.println(1/inv);
	}
} public class Main {

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

	public static void main(String[] ble){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double inv = 0;
		for(int i = 0; i < n; i++){
			double a = sc.nextDouble();
			inv += 1/a;
		}
		System.out.println(1/inv);
	}
}
