import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double ans = 0.0;
		for(int i = 0; i < n; ++i){
			int a = sc.nextInt();
			double tmp = 1.0 / (double)a;
			ans += tmp;
		}
		sc.close();
		ans = 1.0 / ans;
		System.out.println(ans);
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double ans = 0.0;
		for(int i = 0; i < n; ++i){
			int a = sc.nextInt();
			double tmp = 1.0 / (double)a;
			ans += tmp;
		}
		sc.close();
		ans = 1.0 / ans;
		System.out.println(ans);
	}
} Main public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double ans = 0.0;
		for(int i = 0; i < n; ++i){
			int a = sc.nextInt();
			double tmp = 1.0 / (double)a;
			ans += tmp;
		}
		sc.close();
		ans = 1.0 / ans;
		System.out.println(ans);
	} main {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double ans = 0.0;
		for(int i = 0; i < n; ++i){
			int a = sc.nextInt();
			double tmp = 1.0 / (double)a;
			ans += tmp;
		}
		sc.close();
		ans = 1.0 / ans;
		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 = 0.0; ans 0.0 for(int i = 0; i < n; ++i){
			int a = sc.nextInt();
			double tmp = 1.0 / (double)a;
			ans += tmp;
		} int i = 0; int i = 0; i 0 i < n i n ++i ++i i {
			int a = sc.nextInt();
			double tmp = 1.0 / (double)a;
			ans += tmp;
		} {
			int a = sc.nextInt();
			double tmp = 1.0 / (double)a;
			ans += tmp;
		} int a = sc.nextInt(); a sc.nextInt() sc.nextInt sc double tmp = 1.0 / (double)a; tmp 1.0 / (double)a 1.0 (double)a ans += tmp ans tmp sc.close() sc.close sc ans = 1.0 / ans ans 1.0 / ans 1.0 ans 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) throws Exception {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double ans = 0.0;
		for(int i = 0; i < n; ++i){
			int a = sc.nextInt();
			double tmp = 1.0 / (double)a;
			ans += tmp;
		}
		sc.close();
		ans = 1.0 / ans;
		System.out.println(ans);
	}
} public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double ans = 0.0;
		for(int i = 0; i < n; ++i){
			int a = sc.nextInt();
			double tmp = 1.0 / (double)a;
			ans += tmp;
		}
		sc.close();
		ans = 1.0 / ans;
		System.out.println(ans);
	}
} Main
import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double ans = 0.0;
		for(int i = 0; i < n; ++i){
			int a = sc.nextInt();
			double tmp = 1.0 / (double)a;
			ans += tmp;
		}
		sc.close();
		ans = 1.0 / ans;
		System.out.println(ans);
	}
}
