import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		 Scanner sc = new Scanner(System.in);
	        int N = sc.nextInt();
	        double ans = 0;
	        double A = 0;
	        double A1 = 0;
			
			for (int i=0; i<N; i++){
				A1 = sc.nextDouble();
				A += (1 / A1);
			}
			
			ans = 1/A;
			
			System.out.println(ans);
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {
		 Scanner sc = new Scanner(System.in);
	        int N = sc.nextInt();
	        double ans = 0;
	        double A = 0;
	        double A1 = 0;
			
			for (int i=0; i<N; i++){
				A1 = sc.nextDouble();
				A += (1 / A1);
			}
			
			ans = 1/A;
			
			System.out.println(ans);
	}
} Main public static void main(String[] args) {
		 Scanner sc = new Scanner(System.in);
	        int N = sc.nextInt();
	        double ans = 0;
	        double A = 0;
	        double A1 = 0;
			
			for (int i=0; i<N; i++){
				A1 = sc.nextDouble();
				A += (1 / A1);
			}
			
			ans = 1/A;
			
			System.out.println(ans);
	} main {
		 Scanner sc = new Scanner(System.in);
	        int N = sc.nextInt();
	        double ans = 0;
	        double A = 0;
	        double A1 = 0;
			
			for (int i=0; i<N; i++){
				A1 = sc.nextDouble();
				A += (1 / A1);
			}
			
			ans = 1/A;
			
			System.out.println(ans);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc double ans = 0; ans 0 double A = 0; A 0 double A1 = 0; A1 0 for (int i=0; i<N; i++){
				A1 = sc.nextDouble();
				A += (1 / A1);
			} int i=0; int i=0; i 0 i<N i N i++ i++ i {
				A1 = sc.nextDouble();
				A += (1 / A1);
			} {
				A1 = sc.nextDouble();
				A += (1 / A1);
			} A1 = sc.nextDouble() A1 sc.nextDouble() sc.nextDouble sc A += (1 / A1) A (1 / A1) 1 A1 ans = 1/A ans 1/A 1 A System.out.println(ans) System.out.println System.out System System.out ans String[] args args public class Main {
	public static void main(String[] args) {
		 Scanner sc = new Scanner(System.in);
	        int N = sc.nextInt();
	        double ans = 0;
	        double A = 0;
	        double A1 = 0;
			
			for (int i=0; i<N; i++){
				A1 = sc.nextDouble();
				A += (1 / A1);
			}
			
			ans = 1/A;
			
			System.out.println(ans);
	}
} public class Main {
	public static void main(String[] args) {
		 Scanner sc = new Scanner(System.in);
	        int N = sc.nextInt();
	        double ans = 0;
	        double A = 0;
	        double A1 = 0;
			
			for (int i=0; i<N; i++){
				A1 = sc.nextDouble();
				A += (1 / A1);
			}
			
			ans = 1/A;
			
			System.out.println(ans);
	}
} Main
import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		 Scanner sc = new Scanner(System.in);
	        int N = sc.nextInt();
	        double ans = 0;
	        double A = 0;
	        double A1 = 0;
			
			for (int i=0; i<N; i++){
				A1 = sc.nextDouble();
				A += (1 / A1);
			}
			
			ans = 1/A;
			
			System.out.println(ans);
	}
}