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();
		int[] input = new int[N];
		for (Integer i = 0; i < input.length; i++) {
			input[i] = sc.nextInt();
		}
		double ans = 0;
		for (int i = 0; i < input.length; i++) {
			ans += (double) 1 / input[i];
		}
		ans = 1 / ans;
		System.out.println(String.format("%.5f", ans));
		sc.close();
	}
} 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();
		int[] input = new int[N];
		for (Integer i = 0; i < input.length; i++) {
			input[i] = sc.nextInt();
		}
		double ans = 0;
		for (int i = 0; i < input.length; i++) {
			ans += (double) 1 / input[i];
		}
		ans = 1 / ans;
		System.out.println(String.format("%.5f", ans));
		sc.close();
	}
} Main public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int[] input = new int[N];
		for (Integer i = 0; i < input.length; i++) {
			input[i] = sc.nextInt();
		}
		double ans = 0;
		for (int i = 0; i < input.length; i++) {
			ans += (double) 1 / input[i];
		}
		ans = 1 / ans;
		System.out.println(String.format("%.5f", ans));
		sc.close();
	} main {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int[] input = new int[N];
		for (Integer i = 0; i < input.length; i++) {
			input[i] = sc.nextInt();
		}
		double ans = 0;
		for (int i = 0; i < input.length; i++) {
			ans += (double) 1 / input[i];
		}
		ans = 1 / ans;
		System.out.println(String.format("%.5f", ans));
		sc.close();
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc int[] input = new int[N]; input new int[N] N for (Integer i = 0; i < input.length; i++) {
			input[i] = sc.nextInt();
		} Integer i = 0; Integer i = 0; i 0 i < input.length i input.length input input.length i++ i++ i {
			input[i] = sc.nextInt();
		} {
			input[i] = sc.nextInt();
		} input[i] = sc.nextInt() input[i] input i sc.nextInt() sc.nextInt sc double ans = 0; ans 0 for (int i = 0; i < input.length; i++) {
			ans += (double) 1 / input[i];
		} int i = 0; int i = 0; i 0 i < input.length i input.length input input.length i++ i++ i {
			ans += (double) 1 / input[i];
		} {
			ans += (double) 1 / input[i];
		} ans += (double) 1 / input[i] ans (double) 1 / input[i] (double) 1 input[i] input i ans = 1 / ans ans 1 / ans 1 ans System.out.println(String.format("%.5f", ans)) System.out.println System.out System System.out String.format("%.5f", ans) String.format String "%.5f" ans sc.close() sc.close sc String[] args args public class Main {

	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int[] input = new int[N];
		for (Integer i = 0; i < input.length; i++) {
			input[i] = sc.nextInt();
		}
		double ans = 0;
		for (int i = 0; i < input.length; i++) {
			ans += (double) 1 / input[i];
		}
		ans = 1 / ans;
		System.out.println(String.format("%.5f", ans));
		sc.close();
	}
} public class Main {

	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int[] input = new int[N];
		for (Integer i = 0; i < input.length; i++) {
			input[i] = sc.nextInt();
		}
		double ans = 0;
		for (int i = 0; i < input.length; i++) {
			ans += (double) 1 / input[i];
		}
		ans = 1 / ans;
		System.out.println(String.format("%.5f", ans));
		sc.close();
	}
} 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();
		int[] input = new int[N];
		for (Integer i = 0; i < input.length; i++) {
			input[i] = sc.nextInt();
		}
		double ans = 0;
		for (int i = 0; i < input.length; i++) {
			ans += (double) 1 / input[i];
		}
		ans = 1 / ans;
		System.out.println(String.format("%.5f", ans));
		sc.close();
	}
}