import java.util.*;
public class Main {
 public static void main(String[] args){
	Scanner sc = new Scanner(System.in);
	int N=sc.nextInt();

	double bunsi=0;
   	for(int i=0;i<N;i++)bunsi+=(1000.0/sc.nextInt());
	System.out.println(1000/bunsi);
 }
} 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 bunsi=0;
   	for(int i=0;i<N;i++)bunsi+=(1000.0/sc.nextInt());
	System.out.println(1000/bunsi);
 }
} Main public static void main(String[] args){
	Scanner sc = new Scanner(System.in);
	int N=sc.nextInt();

	double bunsi=0;
   	for(int i=0;i<N;i++)bunsi+=(1000.0/sc.nextInt());
	System.out.println(1000/bunsi);
 } main {
	Scanner sc = new Scanner(System.in);
	int N=sc.nextInt();

	double bunsi=0;
   	for(int i=0;i<N;i++)bunsi+=(1000.0/sc.nextInt());
	System.out.println(1000/bunsi);
 } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N=sc.nextInt(); N sc.nextInt() sc.nextInt sc double bunsi=0; bunsi 0 for(int i=0;i<N;i++)bunsi+=(1000.0/sc.nextInt()); int i=0; int i=0; i 0 i<N i N i++ i++ i bunsi+=(1000.0/sc.nextInt()); bunsi+=(1000.0/sc.nextInt()) bunsi (1000.0/sc.nextInt()) 1000.0 sc.nextInt() sc.nextInt sc System.out.println(1000/bunsi) System.out.println System.out System System.out 1000/bunsi 1000 bunsi String[] args args public class Main {
 public static void main(String[] args){
	Scanner sc = new Scanner(System.in);
	int N=sc.nextInt();

	double bunsi=0;
   	for(int i=0;i<N;i++)bunsi+=(1000.0/sc.nextInt());
	System.out.println(1000/bunsi);
 }
} public class Main {
 public static void main(String[] args){
	Scanner sc = new Scanner(System.in);
	int N=sc.nextInt();

	double bunsi=0;
   	for(int i=0;i<N;i++)bunsi+=(1000.0/sc.nextInt());
	System.out.println(1000/bunsi);
 }
} Main
import java.util.*;
public class Main {
 public static void main(String[] args){
	Scanner sc = new Scanner(System.in);
	int N=sc.nextInt();

	double bunsi=0;
   	for(int i=0;i<N;i++)bunsi+=(1000.0/sc.nextInt());
	System.out.println(1000/bunsi);
 }
}
