import java.util.Scanner;

class Main{
  public static void main(String[] args){
    Scanner scan=new Scanner(System.in);
    int x=scan.nextInt();
    double Ans=0;
    for(int a=0; a<x; a++){
      Ans+=1.0/scan.nextInt();
    }
    Ans=1/Ans;
    System.out.println(Ans);
  }
} import java.util.Scanner; Scanner java.util class Main{
  public static void main(String[] args){
    Scanner scan=new Scanner(System.in);
    int x=scan.nextInt();
    double Ans=0;
    for(int a=0; a<x; a++){
      Ans+=1.0/scan.nextInt();
    }
    Ans=1/Ans;
    System.out.println(Ans);
  }
} Main public static void main(String[] args){
    Scanner scan=new Scanner(System.in);
    int x=scan.nextInt();
    double Ans=0;
    for(int a=0; a<x; a++){
      Ans+=1.0/scan.nextInt();
    }
    Ans=1/Ans;
    System.out.println(Ans);
  } main {
    Scanner scan=new Scanner(System.in);
    int x=scan.nextInt();
    double Ans=0;
    for(int a=0; a<x; a++){
      Ans+=1.0/scan.nextInt();
    }
    Ans=1/Ans;
    System.out.println(Ans);
  } Scanner scan=new Scanner(System.in); scan new Scanner(System.in) int x=scan.nextInt(); x scan.nextInt() scan.nextInt scan double Ans=0; Ans 0 for(int a=0; a<x; a++){
      Ans+=1.0/scan.nextInt();
    } int a=0; int a=0; a 0 a<x a x a++ a++ a {
      Ans+=1.0/scan.nextInt();
    } {
      Ans+=1.0/scan.nextInt();
    } Ans+=1.0/scan.nextInt() Ans 1.0/scan.nextInt() 1.0 scan.nextInt() scan.nextInt scan Ans=1/Ans Ans 1/Ans 1 Ans System.out.println(Ans) System.out.println System.out System System.out Ans String[] args args
import java.util.Scanner;

class Main{
  public static void main(String[] args){
    Scanner scan=new Scanner(System.in);
    int x=scan.nextInt();
    double Ans=0;
    for(int a=0; a<x; a++){
      Ans+=1.0/scan.nextInt();
    }
    Ans=1/Ans;
    System.out.println(Ans);
  }
}