import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int N = Integer.parseInt(sc.next());
		double[] in = new double[N];
		double sum = 0;
		
		for (int i = 0; i < N; i++) {
			in[i] = Double.parseDouble(sc.next());
			sum += 1 / in[i];
		}
		
		System.out.println(1 / sum);
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int N = Integer.parseInt(sc.next());
		double[] in = new double[N];
		double sum = 0;
		
		for (int i = 0; i < N; i++) {
			in[i] = Double.parseDouble(sc.next());
			sum += 1 / in[i];
		}
		
		System.out.println(1 / sum);
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int N = Integer.parseInt(sc.next());
		double[] in = new double[N];
		double sum = 0;
		
		for (int i = 0; i < N; i++) {
			in[i] = Double.parseDouble(sc.next());
			sum += 1 / in[i];
		}
		
		System.out.println(1 / sum);
	} main {
		Scanner sc = new Scanner(System.in);
		
		int N = Integer.parseInt(sc.next());
		double[] in = new double[N];
		double sum = 0;
		
		for (int i = 0; i < N; i++) {
			in[i] = Double.parseDouble(sc.next());
			sum += 1 / in[i];
		}
		
		System.out.println(1 / sum);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = Integer.parseInt(sc.next()); N Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc double[] in = new double[N]; in new double[N] N double sum = 0; sum 0 for (int i = 0; i < N; i++) {
			in[i] = Double.parseDouble(sc.next());
			sum += 1 / in[i];
		} int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
			in[i] = Double.parseDouble(sc.next());
			sum += 1 / in[i];
		} {
			in[i] = Double.parseDouble(sc.next());
			sum += 1 / in[i];
		} in[i] = Double.parseDouble(sc.next()) in[i] in i Double.parseDouble(sc.next()) Double.parseDouble Double sc.next() sc.next sc sum += 1 / in[i] sum 1 / in[i] 1 in[i] in i System.out.println(1 / sum) System.out.println System.out System System.out 1 / sum 1 sum String[] args args public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int N = Integer.parseInt(sc.next());
		double[] in = new double[N];
		double sum = 0;
		
		for (int i = 0; i < N; i++) {
			in[i] = Double.parseDouble(sc.next());
			sum += 1 / in[i];
		}
		
		System.out.println(1 / sum);
	}
} public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int N = Integer.parseInt(sc.next());
		double[] in = new double[N];
		double sum = 0;
		
		for (int i = 0; i < N; i++) {
			in[i] = Double.parseDouble(sc.next());
			sum += 1 / in[i];
		}
		
		System.out.println(1 / sum);
	}
} Main
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int N = Integer.parseInt(sc.next());
		double[] in = new double[N];
		double sum = 0;
		
		for (int i = 0; i < N; i++) {
			in[i] = Double.parseDouble(sc.next());
			sum += 1 / in[i];
		}
		
		System.out.println(1 / sum);
	}
}
