import java.text.DecimalFormat;
import java.util.*;
public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		double output = 0;
		double addition = 0;
		for(int i = 0;i<N;i++) {
			double under = 1/sc.nextDouble();
			addition += under;
		}
		output = 1/addition;
		System.out.println(output);
		
	}

} import java.text.DecimalFormat; DecimalFormat java.text import java.util.*; util.* java public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		double output = 0;
		double addition = 0;
		for(int i = 0;i<N;i++) {
			double under = 1/sc.nextDouble();
			addition += under;
		}
		output = 1/addition;
		System.out.println(output);
		
	}

} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		double output = 0;
		double addition = 0;
		for(int i = 0;i<N;i++) {
			double under = 1/sc.nextDouble();
			addition += under;
		}
		output = 1/addition;
		System.out.println(output);
		
	} main {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		double output = 0;
		double addition = 0;
		for(int i = 0;i<N;i++) {
			double under = 1/sc.nextDouble();
			addition += under;
		}
		output = 1/addition;
		System.out.println(output);
		
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc double output = 0; output 0 double addition = 0; addition 0 for(int i = 0;i<N;i++) {
			double under = 1/sc.nextDouble();
			addition += under;
		} int i = 0; int i = 0; i 0 i<N i N i++ i++ i {
			double under = 1/sc.nextDouble();
			addition += under;
		} {
			double under = 1/sc.nextDouble();
			addition += under;
		} double under = 1/sc.nextDouble(); under 1/sc.nextDouble() 1 sc.nextDouble() sc.nextDouble sc addition += under addition under output = 1/addition output 1/addition 1 addition System.out.println(output) System.out.println System.out System System.out output String[] args args public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		double output = 0;
		double addition = 0;
		for(int i = 0;i<N;i++) {
			double under = 1/sc.nextDouble();
			addition += under;
		}
		output = 1/addition;
		System.out.println(output);
		
	}

} public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		double output = 0;
		double addition = 0;
		for(int i = 0;i<N;i++) {
			double under = 1/sc.nextDouble();
			addition += under;
		}
		output = 1/addition;
		System.out.println(output);
		
	}

} Main
import java.text.DecimalFormat;
import java.util.*;
public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		double output = 0;
		double addition = 0;
		for(int i = 0;i<N;i++) {
			double under = 1/sc.nextDouble();
			addition += under;
		}
		output = 1/addition;
		System.out.println(output);
		
	}

}
