import java.io.*;
public class Main{
  public static void main(String[] args) throws IOException{
    BufferedReader reader = new BufferedReader( new InputStreamReader(System.in));
    int a = Integer.parseInt(reader.readLine());
   
    double r = 0.0;
    String[] s =  reader.readLine().split(" ");
    for(int i =0;i<a;i++){
      r = r+ 1.0/(double)Integer.parseInt(s[i]);}
 
      System.out.println(1.0/r);}
} import java.io.*; io.* java public class Main{
  public static void main(String[] args) throws IOException{
    BufferedReader reader = new BufferedReader( new InputStreamReader(System.in));
    int a = Integer.parseInt(reader.readLine());
   
    double r = 0.0;
    String[] s =  reader.readLine().split(" ");
    for(int i =0;i<a;i++){
      r = r+ 1.0/(double)Integer.parseInt(s[i]);}
 
      System.out.println(1.0/r);}
} Main public static void main(String[] args) throws IOException{
    BufferedReader reader = new BufferedReader( new InputStreamReader(System.in));
    int a = Integer.parseInt(reader.readLine());
   
    double r = 0.0;
    String[] s =  reader.readLine().split(" ");
    for(int i =0;i<a;i++){
      r = r+ 1.0/(double)Integer.parseInt(s[i]);}
 
      System.out.println(1.0/r);} main {
    BufferedReader reader = new BufferedReader( new InputStreamReader(System.in));
    int a = Integer.parseInt(reader.readLine());
   
    double r = 0.0;
    String[] s =  reader.readLine().split(" ");
    for(int i =0;i<a;i++){
      r = r+ 1.0/(double)Integer.parseInt(s[i]);}
 
      System.out.println(1.0/r);} BufferedReader reader = new BufferedReader( new InputStreamReader(System.in)); reader new BufferedReader( new InputStreamReader(System.in)) int a = Integer.parseInt(reader.readLine()); a Integer.parseInt(reader.readLine()) Integer.parseInt Integer reader.readLine() reader.readLine reader double r = 0.0; r 0.0 String[] s =  reader.readLine().split(" "); s reader.readLine().split(" ") reader.readLine().split reader.readLine() reader.readLine reader " " for(int i =0;i<a;i++){
      r = r+ 1.0/(double)Integer.parseInt(s[i]);} int i =0; int i =0; i 0 i<a i a i++ i++ i {
      r = r+ 1.0/(double)Integer.parseInt(s[i]);} {
      r = r+ 1.0/(double)Integer.parseInt(s[i]);} r = r+ 1.0/(double)Integer.parseInt(s[i]) r r+ 1.0/(double)Integer.parseInt(s[i]) r 1.0/(double)Integer.parseInt(s[i]) 1.0 (double)Integer.parseInt(s[i]) Integer.parseInt Integer s[i] s i System.out.println(1.0/r) System.out.println System.out System System.out 1.0/r 1.0 r String[] args args public class Main{
  public static void main(String[] args) throws IOException{
    BufferedReader reader = new BufferedReader( new InputStreamReader(System.in));
    int a = Integer.parseInt(reader.readLine());
   
    double r = 0.0;
    String[] s =  reader.readLine().split(" ");
    for(int i =0;i<a;i++){
      r = r+ 1.0/(double)Integer.parseInt(s[i]);}
 
      System.out.println(1.0/r);}
} public class Main{
  public static void main(String[] args) throws IOException{
    BufferedReader reader = new BufferedReader( new InputStreamReader(System.in));
    int a = Integer.parseInt(reader.readLine());
   
    double r = 0.0;
    String[] s =  reader.readLine().split(" ");
    for(int i =0;i<a;i++){
      r = r+ 1.0/(double)Integer.parseInt(s[i]);}
 
      System.out.println(1.0/r);}
} Main
import java.io.*;
public class Main{
  public static void main(String[] args) throws IOException{
    BufferedReader reader = new BufferedReader( new InputStreamReader(System.in));
    int a = Integer.parseInt(reader.readLine());
   
    double r = 0.0;
    String[] s =  reader.readLine().split(" ");
    for(int i =0;i<a;i++){
      r = r+ 1.0/(double)Integer.parseInt(s[i]);}
 
      System.out.println(1.0/r);}
}