import java.util.*;
class Main
{
  public static void main(String []args)
  {
    Scanner sc=new Scanner(System.in);
    int a=sc.nextInt();
    int z=0;
    double d=0.0;
    for(int i=0;i<a;i++)
    {
      z=sc.nextInt();
      d=d+1.0/z;
    }
    d=1/d;
    System.out.println(d);
    }
} import java.util.*; util.* java class Main
{
  public static void main(String []args)
  {
    Scanner sc=new Scanner(System.in);
    int a=sc.nextInt();
    int z=0;
    double d=0.0;
    for(int i=0;i<a;i++)
    {
      z=sc.nextInt();
      d=d+1.0/z;
    }
    d=1/d;
    System.out.println(d);
    }
} Main public static void main(String []args)
  {
    Scanner sc=new Scanner(System.in);
    int a=sc.nextInt();
    int z=0;
    double d=0.0;
    for(int i=0;i<a;i++)
    {
      z=sc.nextInt();
      d=d+1.0/z;
    }
    d=1/d;
    System.out.println(d);
    } main {
    Scanner sc=new Scanner(System.in);
    int a=sc.nextInt();
    int z=0;
    double d=0.0;
    for(int i=0;i<a;i++)
    {
      z=sc.nextInt();
      d=d+1.0/z;
    }
    d=1/d;
    System.out.println(d);
    } Scanner sc=new Scanner(System.in); sc new Scanner(System.in) int a=sc.nextInt(); a sc.nextInt() sc.nextInt sc int z=0; z 0 double d=0.0; d 0.0 for(int i=0;i<a;i++)
    {
      z=sc.nextInt();
      d=d+1.0/z;
    } int i=0; int i=0; i 0 i<a i a i++ i++ i {
      z=sc.nextInt();
      d=d+1.0/z;
    } {
      z=sc.nextInt();
      d=d+1.0/z;
    } z=sc.nextInt() z sc.nextInt() sc.nextInt sc d=d+1.0/z d d+1.0/z d 1.0/z 1.0 z d=1/d d 1/d 1 d System.out.println(d) System.out.println System.out System System.out d String []args args
import java.util.*;
class Main
{
  public static void main(String []args)
  {
    Scanner sc=new Scanner(System.in);
    int a=sc.nextInt();
    int z=0;
    double d=0.0;
    for(int i=0;i<a;i++)
    {
      z=sc.nextInt();
      d=d+1.0/z;
    }
    d=1/d;
    System.out.println(d);
    }
}