import java.util.*;

public class Main {

	public static void main(String[] args) {
		
		Scanner sc = new Scanner(System.in);
		
		int n = sc.nextInt();
		double a = 0;
		
		for(int i = 0; i < n; i++) a += 1.0 / sc.nextInt();

		System.out.println(1 / a);
	}
} import java.util.*; util.* java public class Main {

	public static void main(String[] args) {
		
		Scanner sc = new Scanner(System.in);
		
		int n = sc.nextInt();
		double a = 0;
		
		for(int i = 0; i < n; i++) a += 1.0 / sc.nextInt();

		System.out.println(1 / a);
	}
} Main public static void main(String[] args) {
		
		Scanner sc = new Scanner(System.in);
		
		int n = sc.nextInt();
		double a = 0;
		
		for(int i = 0; i < n; i++) a += 1.0 / sc.nextInt();

		System.out.println(1 / a);
	} main {
		
		Scanner sc = new Scanner(System.in);
		
		int n = sc.nextInt();
		double a = 0;
		
		for(int i = 0; i < n; i++) a += 1.0 / sc.nextInt();

		System.out.println(1 / a);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc double a = 0; a 0 for(int i = 0; i < n; i++) a += 1.0 / sc.nextInt(); int i = 0; int i = 0; i 0 i < n i n i++ i++ i a += 1.0 / sc.nextInt(); a += 1.0 / sc.nextInt() a 1.0 / sc.nextInt() 1.0 sc.nextInt() sc.nextInt sc System.out.println(1 / a) System.out.println System.out System System.out 1 / a 1 a String[] args args public class Main {

	public static void main(String[] args) {
		
		Scanner sc = new Scanner(System.in);
		
		int n = sc.nextInt();
		double a = 0;
		
		for(int i = 0; i < n; i++) a += 1.0 / sc.nextInt();

		System.out.println(1 / a);
	}
} public class Main {

	public static void main(String[] args) {
		
		Scanner sc = new Scanner(System.in);
		
		int n = sc.nextInt();
		double a = 0;
		
		for(int i = 0; i < n; i++) a += 1.0 / sc.nextInt();

		System.out.println(1 / a);
	}
} Main
import java.util.*;

public class Main {

	public static void main(String[] args) {
		
		Scanner sc = new Scanner(System.in);
		
		int n = sc.nextInt();
		double a = 0;
		
		for(int i = 0; i < n; i++) a += 1.0 / sc.nextInt();

		System.out.println(1 / a);
	}
}