import java.util.*;

public class Main {
	static int N=26;
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		N = sc.nextInt();
		int data[] =new int[N];
		double t=0;
	
		for(int i=0;i<data.length;i++) {
			data[i]=sc.nextInt();
			
			t += (double)1.0/data[i];
		}
		double ans =( double)1.0/t;
	System.out.println(ans);

		

	}

} import java.util.*; util.* java public class Main {
	static int N=26;
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		N = sc.nextInt();
		int data[] =new int[N];
		double t=0;
	
		for(int i=0;i<data.length;i++) {
			data[i]=sc.nextInt();
			
			t += (double)1.0/data[i];
		}
		double ans =( double)1.0/t;
	System.out.println(ans);

		

	}

} Main static int N=26; N 26 public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		N = sc.nextInt();
		int data[] =new int[N];
		double t=0;
	
		for(int i=0;i<data.length;i++) {
			data[i]=sc.nextInt();
			
			t += (double)1.0/data[i];
		}
		double ans =( double)1.0/t;
	System.out.println(ans);

		

	} main {
		Scanner sc = new Scanner(System.in);
		N = sc.nextInt();
		int data[] =new int[N];
		double t=0;
	
		for(int i=0;i<data.length;i++) {
			data[i]=sc.nextInt();
			
			t += (double)1.0/data[i];
		}
		double ans =( double)1.0/t;
	System.out.println(ans);

		

	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) N = sc.nextInt() N sc.nextInt() sc.nextInt sc int data[] =new int[N]; data new int[N] N double t=0; t 0 for(int i=0;i<data.length;i++) {
			data[i]=sc.nextInt();
			
			t += (double)1.0/data[i];
		} int i=0; int i=0; i 0 i<data.length i data.length data data.length i++ i++ i {
			data[i]=sc.nextInt();
			
			t += (double)1.0/data[i];
		} {
			data[i]=sc.nextInt();
			
			t += (double)1.0/data[i];
		} data[i]=sc.nextInt() data[i] data i sc.nextInt() sc.nextInt sc t += (double)1.0/data[i] t (double)1.0/data[i] (double)1.0 data[i] data i double ans =( double)1.0/t; ans ( double)1.0/t ( double)1.0 t System.out.println(ans) System.out.println System.out System System.out ans String args[] args public class Main {
	static int N=26;
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		N = sc.nextInt();
		int data[] =new int[N];
		double t=0;
	
		for(int i=0;i<data.length;i++) {
			data[i]=sc.nextInt();
			
			t += (double)1.0/data[i];
		}
		double ans =( double)1.0/t;
	System.out.println(ans);

		

	}

} public class Main {
	static int N=26;
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		N = sc.nextInt();
		int data[] =new int[N];
		double t=0;
	
		for(int i=0;i<data.length;i++) {
			data[i]=sc.nextInt();
			
			t += (double)1.0/data[i];
		}
		double ans =( double)1.0/t;
	System.out.println(ans);

		

	}

} Main
import java.util.*;

public class Main {
	static int N=26;
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		N = sc.nextInt();
		int data[] =new int[N];
		double t=0;
	
		for(int i=0;i<data.length;i++) {
			data[i]=sc.nextInt();
			
			t += (double)1.0/data[i];
		}
		double ans =( double)1.0/t;
	System.out.println(ans);

		

	}

}