
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner scn=new Scanner(System.in);
		int n=scn.nextInt();
		double []arr=new double [n];
		double x=1;
		double sum=0;
		for(int i=0;i<n;i++) {
			arr[i]=scn.nextInt();
			arr[i]=(double)(x/arr[i]);
			sum+=arr[i];
		}
		System.out.println(x/sum);
	}

} import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner scn=new Scanner(System.in);
		int n=scn.nextInt();
		double []arr=new double [n];
		double x=1;
		double sum=0;
		for(int i=0;i<n;i++) {
			arr[i]=scn.nextInt();
			arr[i]=(double)(x/arr[i]);
			sum+=arr[i];
		}
		System.out.println(x/sum);
	}

} Main public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner scn=new Scanner(System.in);
		int n=scn.nextInt();
		double []arr=new double [n];
		double x=1;
		double sum=0;
		for(int i=0;i<n;i++) {
			arr[i]=scn.nextInt();
			arr[i]=(double)(x/arr[i]);
			sum+=arr[i];
		}
		System.out.println(x/sum);
	} main {
		// TODO Auto-generated method stub
		Scanner scn=new Scanner(System.in);
		int n=scn.nextInt();
		double []arr=new double [n];
		double x=1;
		double sum=0;
		for(int i=0;i<n;i++) {
			arr[i]=scn.nextInt();
			arr[i]=(double)(x/arr[i]);
			sum+=arr[i];
		}
		System.out.println(x/sum);
	} Scanner scn=new Scanner(System.in); scn new Scanner(System.in) int n=scn.nextInt(); n scn.nextInt() scn.nextInt scn double []arr=new double [n]; arr new double [n] n double x=1; x 1 double sum=0; sum 0 for(int i=0;i<n;i++) {
			arr[i]=scn.nextInt();
			arr[i]=(double)(x/arr[i]);
			sum+=arr[i];
		} int i=0; int i=0; i 0 i<n i n i++ i++ i {
			arr[i]=scn.nextInt();
			arr[i]=(double)(x/arr[i]);
			sum+=arr[i];
		} {
			arr[i]=scn.nextInt();
			arr[i]=(double)(x/arr[i]);
			sum+=arr[i];
		} arr[i]=scn.nextInt() arr[i] arr i scn.nextInt() scn.nextInt scn arr[i]=(double)(x/arr[i]) arr[i] arr i (double)(x/arr[i]) x arr[i] arr i sum+=arr[i] sum arr[i] arr i System.out.println(x/sum) System.out.println System.out System System.out x/sum x sum String[] args args public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner scn=new Scanner(System.in);
		int n=scn.nextInt();
		double []arr=new double [n];
		double x=1;
		double sum=0;
		for(int i=0;i<n;i++) {
			arr[i]=scn.nextInt();
			arr[i]=(double)(x/arr[i]);
			sum+=arr[i];
		}
		System.out.println(x/sum);
	}

} public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner scn=new Scanner(System.in);
		int n=scn.nextInt();
		double []arr=new double [n];
		double x=1;
		double sum=0;
		for(int i=0;i<n;i++) {
			arr[i]=scn.nextInt();
			arr[i]=(double)(x/arr[i]);
			sum+=arr[i];
		}
		System.out.println(x/sum);
	}

} Main

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner scn=new Scanner(System.in);
		int n=scn.nextInt();
		double []arr=new double [n];
		double x=1;
		double sum=0;
		for(int i=0;i<n;i++) {
			arr[i]=scn.nextInt();
			arr[i]=(double)(x/arr[i]);
			sum+=arr[i];
		}
		System.out.println(x/sum);
	}

}
