import java.util.Scanner;

public class Main {

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

	}

} 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();
		int a = 0;
		double b = 0;
		for(int i=0; i<N; i++) {
			a = sc.nextInt();
			b += 1.0/a;
		}
		sc.close();
		double c = 1/b;
		System.out.println(c);

	}

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

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

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

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

	}

} public class Main {

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

	}

} Main
import java.util.Scanner;

public class Main {

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

	}

}