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