import java.util.Scanner;
public class Main{
 public static void main(String[] args){
   Scanner sc = new Scanner(System.in);
   int N = sc.nextInt();
   double[] Si = new double[N];
   for(int i = 0; i<N;i++)
   {
		Si[i] = sc.nextDouble();       
   }
   double sum_re = 0.0;
   for(int i = 0; i<N;i++)
   {
		sum_re += 1/Si[i];
   }
   System.out.println(1/sum_re);
 }
} import java.util.Scanner; Scanner java.util public class Main{
 public static void main(String[] args){
   Scanner sc = new Scanner(System.in);
   int N = sc.nextInt();
   double[] Si = new double[N];
   for(int i = 0; i<N;i++)
   {
		Si[i] = sc.nextDouble();       
   }
   double sum_re = 0.0;
   for(int i = 0; i<N;i++)
   {
		sum_re += 1/Si[i];
   }
   System.out.println(1/sum_re);
 }
} Main public static void main(String[] args){
   Scanner sc = new Scanner(System.in);
   int N = sc.nextInt();
   double[] Si = new double[N];
   for(int i = 0; i<N;i++)
   {
		Si[i] = sc.nextDouble();       
   }
   double sum_re = 0.0;
   for(int i = 0; i<N;i++)
   {
		sum_re += 1/Si[i];
   }
   System.out.println(1/sum_re);
 } main {
   Scanner sc = new Scanner(System.in);
   int N = sc.nextInt();
   double[] Si = new double[N];
   for(int i = 0; i<N;i++)
   {
		Si[i] = sc.nextDouble();       
   }
   double sum_re = 0.0;
   for(int i = 0; i<N;i++)
   {
		sum_re += 1/Si[i];
   }
   System.out.println(1/sum_re);
 } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc double[] Si = new double[N]; Si new double[N] N for(int i = 0; i<N;i++)
   {
		Si[i] = sc.nextDouble();       
   } int i = 0; int i = 0; i 0 i<N i N i++ i++ i {
		Si[i] = sc.nextDouble();       
   } {
		Si[i] = sc.nextDouble();       
   } Si[i] = sc.nextDouble() Si[i] Si i sc.nextDouble() sc.nextDouble sc double sum_re = 0.0; sum_re 0.0 for(int i = 0; i<N;i++)
   {
		sum_re += 1/Si[i];
   } int i = 0; int i = 0; i 0 i<N i N i++ i++ i {
		sum_re += 1/Si[i];
   } {
		sum_re += 1/Si[i];
   } sum_re += 1/Si[i] sum_re 1/Si[i] 1 Si[i] Si i System.out.println(1/sum_re) System.out.println System.out System System.out 1/sum_re 1 sum_re String[] args args public class Main{
 public static void main(String[] args){
   Scanner sc = new Scanner(System.in);
   int N = sc.nextInt();
   double[] Si = new double[N];
   for(int i = 0; i<N;i++)
   {
		Si[i] = sc.nextDouble();       
   }
   double sum_re = 0.0;
   for(int i = 0; i<N;i++)
   {
		sum_re += 1/Si[i];
   }
   System.out.println(1/sum_re);
 }
} public class Main{
 public static void main(String[] args){
   Scanner sc = new Scanner(System.in);
   int N = sc.nextInt();
   double[] Si = new double[N];
   for(int i = 0; i<N;i++)
   {
		Si[i] = sc.nextDouble();       
   }
   double sum_re = 0.0;
   for(int i = 0; i<N;i++)
   {
		sum_re += 1/Si[i];
   }
   System.out.println(1/sum_re);
 }
} Main
import java.util.Scanner;
public class Main{
 public static void main(String[] args){
   Scanner sc = new Scanner(System.in);
   int N = sc.nextInt();
   double[] Si = new double[N];
   for(int i = 0; i<N;i++)
   {
		Si[i] = sc.nextDouble();       
   }
   double sum_re = 0.0;
   for(int i = 0; i<N;i++)
   {
		sum_re += 1/Si[i];
   }
   System.out.println(1/sum_re);
 }
}