import java.util.Scanner;

public class Main {
	public static void main(String[] args){
		try(Scanner sc = new Scanner(System.in)) {

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

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

			int n = sc.nextInt();
			
			double[] a = new double[n];
			for(int i = 0 ; i < n ; i++ ) {
				a[i] = sc.nextDouble();
			}
			
			double base = 0;
			for(int i = 0 ; i < n ; i++ ) {
				base += 1f/a[i];
			}
			
			double answer = 1f / base;
			System.out.println(answer);
		}
	} main {
		try(Scanner sc = new Scanner(System.in)) {

			int n = sc.nextInt();
			
			double[] a = new double[n];
			for(int i = 0 ; i < n ; i++ ) {
				a[i] = sc.nextDouble();
			}
			
			double base = 0;
			for(int i = 0 ; i < n ; i++ ) {
				base += 1f/a[i];
			}
			
			double answer = 1f / base;
			System.out.println(answer);
		}
	} try(Scanner sc = new Scanner(System.in)) {

			int n = sc.nextInt();
			
			double[] a = new double[n];
			for(int i = 0 ; i < n ; i++ ) {
				a[i] = sc.nextDouble();
			}
			
			double base = 0;
			for(int i = 0 ; i < n ; i++ ) {
				base += 1f/a[i];
			}
			
			double answer = 1f / base;
			System.out.println(answer);
		} {

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

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

			int n = sc.nextInt();
			
			double[] a = new double[n];
			for(int i = 0 ; i < n ; i++ ) {
				a[i] = sc.nextDouble();
			}
			
			double base = 0;
			for(int i = 0 ; i < n ; i++ ) {
				base += 1f/a[i];
			}
			
			double answer = 1f / base;
			System.out.println(answer);
		}
	}
} Main
import java.util.Scanner;

public class Main {
	public static void main(String[] args){
		try(Scanner sc = new Scanner(System.in)) {

			int n = sc.nextInt();
			
			double[] a = new double[n];
			for(int i = 0 ; i < n ; i++ ) {
				a[i] = sc.nextDouble();
			}
			
			double base = 0;
			for(int i = 0 ; i < n ; i++ ) {
				base += 1f/a[i];
			}
			
			double answer = 1f / base;
			System.out.println(answer);
		}
	}
}
