

import java.io.BufferedReader;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.List;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Scanner;
import java.util.Set;
import java.util.StringTokenizer;


public class Main {
	 static class FastReader 
	    { 
	        BufferedReader br; 
	        StringTokenizer st; 
	       
	  
	        public FastReader() 
	        { 
	            br = new BufferedReader(new
	                     InputStreamReader(System.in)); 
	        } 
	  
	        String next() 
	        { 
	            while (st == null || !st.hasMoreTokens()) 
	            { 
	                try
	                { 
	                    st = new StringTokenizer(br.readLine()); 
	                } 
	                catch (IOException  e) 
	                { 
	                    e.printStackTrace(); 
	                } 
	            } 
	            return st.nextToken();
	            
	        } 
	  
	        int nextInt() 
	        { 
	            return Integer.parseInt(next()); 
	        } 
	  
	        long nextLong() 
	        { 
	            return Long.parseLong(next()); 
	        } 
	  
	        double nextDouble() 
	        { 
	            return Double.parseDouble(next()); 
	        } 
	  
	        String nextLine() 
	        { 
	            String str = ""; 
	            try
	            { 
	                str = br.readLine(); 
	            } 
	            catch (IOException e) 
	            { 
	                e.printStackTrace(); 
	            } 
	            return str; 
	        }

			public Character charAt(int i) {
				// TODO Auto-generated method stub
				return null;
			} 
	    } 
 
	public static void main(String[] args)throws IOException  {
		FastReader s=new FastReader();		
		int n =  s.nextInt();
		int[] ar = new int[n];
		double sum=0;
		for(int i=0;i<n;i++) {
			ar[i]=s.nextInt();
			sum+=(double)1/ar[i];
		}
		System.out.println(1/sum);
		
		
	}
	
} import java.io.BufferedReader; BufferedReader java.io import java.util.ArrayList; ArrayList java.util import java.util.Arrays; Arrays java.util import java.util.Collections; Collections java.util import java.util.HashSet; HashSet java.util import java.util.List; List java.util import java.io.IOException; IOException java.io import java.io.InputStreamReader; InputStreamReader java.io import java.util.Scanner; Scanner java.util import java.util.Set; Set java.util import java.util.StringTokenizer; StringTokenizer java.util public class Main {
	 static class FastReader 
	    { 
	        BufferedReader br; 
	        StringTokenizer st; 
	       
	  
	        public FastReader() 
	        { 
	            br = new BufferedReader(new
	                     InputStreamReader(System.in)); 
	        } 
	  
	        String next() 
	        { 
	            while (st == null || !st.hasMoreTokens()) 
	            { 
	                try
	                { 
	                    st = new StringTokenizer(br.readLine()); 
	                } 
	                catch (IOException  e) 
	                { 
	                    e.printStackTrace(); 
	                } 
	            } 
	            return st.nextToken();
	            
	        } 
	  
	        int nextInt() 
	        { 
	            return Integer.parseInt(next()); 
	        } 
	  
	        long nextLong() 
	        { 
	            return Long.parseLong(next()); 
	        } 
	  
	        double nextDouble() 
	        { 
	            return Double.parseDouble(next()); 
	        } 
	  
	        String nextLine() 
	        { 
	            String str = ""; 
	            try
	            { 
	                str = br.readLine(); 
	            } 
	            catch (IOException e) 
	            { 
	                e.printStackTrace(); 
	            } 
	            return str; 
	        }

			public Character charAt(int i) {
				// TODO Auto-generated method stub
				return null;
			} 
	    } 
 
	public static void main(String[] args)throws IOException  {
		FastReader s=new FastReader();		
		int n =  s.nextInt();
		int[] ar = new int[n];
		double sum=0;
		for(int i=0;i<n;i++) {
			ar[i]=s.nextInt();
			sum+=(double)1/ar[i];
		}
		System.out.println(1/sum);
		
		
	}
	
} Main public static void main(String[] args)throws IOException  {
		FastReader s=new FastReader();		
		int n =  s.nextInt();
		int[] ar = new int[n];
		double sum=0;
		for(int i=0;i<n;i++) {
			ar[i]=s.nextInt();
			sum+=(double)1/ar[i];
		}
		System.out.println(1/sum);
		
		
	} main {
		FastReader s=new FastReader();		
		int n =  s.nextInt();
		int[] ar = new int[n];
		double sum=0;
		for(int i=0;i<n;i++) {
			ar[i]=s.nextInt();
			sum+=(double)1/ar[i];
		}
		System.out.println(1/sum);
		
		
	} FastReader s=new FastReader(); s new FastReader() int n =  s.nextInt(); n s.nextInt() s.nextInt s int[] ar = new int[n]; ar new int[n] n double sum=0; sum 0 for(int i=0;i<n;i++) {
			ar[i]=s.nextInt();
			sum+=(double)1/ar[i];
		} int i=0; int i=0; i 0 i<n i n i++ i++ i {
			ar[i]=s.nextInt();
			sum+=(double)1/ar[i];
		} {
			ar[i]=s.nextInt();
			sum+=(double)1/ar[i];
		} ar[i]=s.nextInt() ar[i] ar i s.nextInt() s.nextInt s sum+=(double)1/ar[i] sum (double)1/ar[i] (double)1 ar[i] ar i System.out.println(1/sum) System.out.println System.out System System.out 1/sum 1 sum String[] args args static class FastReader 
	    { 
	        BufferedReader br; 
	        StringTokenizer st; 
	       
	  
	        public FastReader() 
	        { 
	            br = new BufferedReader(new
	                     InputStreamReader(System.in)); 
	        } 
	  
	        String next() 
	        { 
	            while (st == null || !st.hasMoreTokens()) 
	            { 
	                try
	                { 
	                    st = new StringTokenizer(br.readLine()); 
	                } 
	                catch (IOException  e) 
	                { 
	                    e.printStackTrace(); 
	                } 
	            } 
	            return st.nextToken();
	            
	        } 
	  
	        int nextInt() 
	        { 
	            return Integer.parseInt(next()); 
	        } 
	  
	        long nextLong() 
	        { 
	            return Long.parseLong(next()); 
	        } 
	  
	        double nextDouble() 
	        { 
	            return Double.parseDouble(next()); 
	        } 
	  
	        String nextLine() 
	        { 
	            String str = ""; 
	            try
	            { 
	                str = br.readLine(); 
	            } 
	            catch (IOException e) 
	            { 
	                e.printStackTrace(); 
	            } 
	            return str; 
	        }

			public Character charAt(int i) {
				// TODO Auto-generated method stub
				return null;
			} 
	    } FastReader BufferedReader br; br StringTokenizer st; st public FastReader() 
	        { 
	            br = new BufferedReader(new
	                     InputStreamReader(System.in)); 
	        } FastReader { 
	            br = new BufferedReader(new
	                     InputStreamReader(System.in)); 
	        } br = new BufferedReader(new
	                     InputStreamReader(System.in)) br new BufferedReader(new
	                     InputStreamReader(System.in)) String next() 
	        { 
	            while (st == null || !st.hasMoreTokens()) 
	            { 
	                try
	                { 
	                    st = new StringTokenizer(br.readLine()); 
	                } 
	                catch (IOException  e) 
	                { 
	                    e.printStackTrace(); 
	                } 
	            } 
	            return st.nextToken();
	            
	        } next { 
	            while (st == null || !st.hasMoreTokens()) 
	            { 
	                try
	                { 
	                    st = new StringTokenizer(br.readLine()); 
	                } 
	                catch (IOException  e) 
	                { 
	                    e.printStackTrace(); 
	                } 
	            } 
	            return st.nextToken();
	            
	        } while (st == null || !st.hasMoreTokens()) 
	            { 
	                try
	                { 
	                    st = new StringTokenizer(br.readLine()); 
	                } 
	                catch (IOException  e) 
	                { 
	                    e.printStackTrace(); 
	                } 
	            } st == null || !st.hasMoreTokens() st == null st null !st.hasMoreTokens() st.hasMoreTokens() st.hasMoreTokens st { 
	                try
	                { 
	                    st = new StringTokenizer(br.readLine()); 
	                } 
	                catch (IOException  e) 
	                { 
	                    e.printStackTrace(); 
	                } 
	            } try
	                { 
	                    st = new StringTokenizer(br.readLine()); 
	                } 
	                catch (IOException  e) 
	                { 
	                    e.printStackTrace(); 
	                } catch (IOException  e) 
	                { 
	                    e.printStackTrace(); 
	                } IOException  e { 
	                    e.printStackTrace(); 
	                } e.printStackTrace() e.printStackTrace e { 
	                    st = new StringTokenizer(br.readLine()); 
	                } st = new StringTokenizer(br.readLine()) st new StringTokenizer(br.readLine()) return st.nextToken(); st.nextToken() st.nextToken st int nextInt() 
	        { 
	            return Integer.parseInt(next()); 
	        } nextInt { 
	            return Integer.parseInt(next()); 
	        } return Integer.parseInt(next()); Integer.parseInt(next()) Integer.parseInt Integer next() next long nextLong() 
	        { 
	            return Long.parseLong(next()); 
	        } nextLong { 
	            return Long.parseLong(next()); 
	        } return Long.parseLong(next()); Long.parseLong(next()) Long.parseLong Long next() next double nextDouble() 
	        { 
	            return Double.parseDouble(next()); 
	        } nextDouble { 
	            return Double.parseDouble(next()); 
	        } return Double.parseDouble(next()); Double.parseDouble(next()) Double.parseDouble Double next() next String nextLine() 
	        { 
	            String str = ""; 
	            try
	            { 
	                str = br.readLine(); 
	            } 
	            catch (IOException e) 
	            { 
	                e.printStackTrace(); 
	            } 
	            return str; 
	        } nextLine { 
	            String str = ""; 
	            try
	            { 
	                str = br.readLine(); 
	            } 
	            catch (IOException e) 
	            { 
	                e.printStackTrace(); 
	            } 
	            return str; 
	        } String str = ""; str "" try
	            { 
	                str = br.readLine(); 
	            } 
	            catch (IOException e) 
	            { 
	                e.printStackTrace(); 
	            } catch (IOException e) 
	            { 
	                e.printStackTrace(); 
	            } IOException e { 
	                e.printStackTrace(); 
	            } e.printStackTrace() e.printStackTrace e { 
	                str = br.readLine(); 
	            } str = br.readLine() str br.readLine() br.readLine br return str; str public Character charAt(int i) {
				// TODO Auto-generated method stub
				return null;
			} charAt {
				// TODO Auto-generated method stub
				return null;
			} return null; null int i i public class Main {
	 static class FastReader 
	    { 
	        BufferedReader br; 
	        StringTokenizer st; 
	       
	  
	        public FastReader() 
	        { 
	            br = new BufferedReader(new
	                     InputStreamReader(System.in)); 
	        } 
	  
	        String next() 
	        { 
	            while (st == null || !st.hasMoreTokens()) 
	            { 
	                try
	                { 
	                    st = new StringTokenizer(br.readLine()); 
	                } 
	                catch (IOException  e) 
	                { 
	                    e.printStackTrace(); 
	                } 
	            } 
	            return st.nextToken();
	            
	        } 
	  
	        int nextInt() 
	        { 
	            return Integer.parseInt(next()); 
	        } 
	  
	        long nextLong() 
	        { 
	            return Long.parseLong(next()); 
	        } 
	  
	        double nextDouble() 
	        { 
	            return Double.parseDouble(next()); 
	        } 
	  
	        String nextLine() 
	        { 
	            String str = ""; 
	            try
	            { 
	                str = br.readLine(); 
	            } 
	            catch (IOException e) 
	            { 
	                e.printStackTrace(); 
	            } 
	            return str; 
	        }

			public Character charAt(int i) {
				// TODO Auto-generated method stub
				return null;
			} 
	    } 
 
	public static void main(String[] args)throws IOException  {
		FastReader s=new FastReader();		
		int n =  s.nextInt();
		int[] ar = new int[n];
		double sum=0;
		for(int i=0;i<n;i++) {
			ar[i]=s.nextInt();
			sum+=(double)1/ar[i];
		}
		System.out.println(1/sum);
		
		
	}
	
} public class Main {
	 static class FastReader 
	    { 
	        BufferedReader br; 
	        StringTokenizer st; 
	       
	  
	        public FastReader() 
	        { 
	            br = new BufferedReader(new
	                     InputStreamReader(System.in)); 
	        } 
	  
	        String next() 
	        { 
	            while (st == null || !st.hasMoreTokens()) 
	            { 
	                try
	                { 
	                    st = new StringTokenizer(br.readLine()); 
	                } 
	                catch (IOException  e) 
	                { 
	                    e.printStackTrace(); 
	                } 
	            } 
	            return st.nextToken();
	            
	        } 
	  
	        int nextInt() 
	        { 
	            return Integer.parseInt(next()); 
	        } 
	  
	        long nextLong() 
	        { 
	            return Long.parseLong(next()); 
	        } 
	  
	        double nextDouble() 
	        { 
	            return Double.parseDouble(next()); 
	        } 
	  
	        String nextLine() 
	        { 
	            String str = ""; 
	            try
	            { 
	                str = br.readLine(); 
	            } 
	            catch (IOException e) 
	            { 
	                e.printStackTrace(); 
	            } 
	            return str; 
	        }

			public Character charAt(int i) {
				// TODO Auto-generated method stub
				return null;
			} 
	    } 
 
	public static void main(String[] args)throws IOException  {
		FastReader s=new FastReader();		
		int n =  s.nextInt();
		int[] ar = new int[n];
		double sum=0;
		for(int i=0;i<n;i++) {
			ar[i]=s.nextInt();
			sum+=(double)1/ar[i];
		}
		System.out.println(1/sum);
		
		
	}
	
} Main


import java.io.BufferedReader;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.List;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Scanner;
import java.util.Set;
import java.util.StringTokenizer;


public class Main {
	 static class FastReader 
	    { 
	        BufferedReader br; 
	        StringTokenizer st; 
	       
	  
	        public FastReader() 
	        { 
	            br = new BufferedReader(new
	                     InputStreamReader(System.in)); 
	        } 
	  
	        String next() 
	        { 
	            while (st == null || !st.hasMoreTokens()) 
	            { 
	                try
	                { 
	                    st = new StringTokenizer(br.readLine()); 
	                } 
	                catch (IOException  e) 
	                { 
	                    e.printStackTrace(); 
	                } 
	            } 
	            return st.nextToken();
	            
	        } 
	  
	        int nextInt() 
	        { 
	            return Integer.parseInt(next()); 
	        } 
	  
	        long nextLong() 
	        { 
	            return Long.parseLong(next()); 
	        } 
	  
	        double nextDouble() 
	        { 
	            return Double.parseDouble(next()); 
	        } 
	  
	        String nextLine() 
	        { 
	            String str = ""; 
	            try
	            { 
	                str = br.readLine(); 
	            } 
	            catch (IOException e) 
	            { 
	                e.printStackTrace(); 
	            } 
	            return str; 
	        }

			public Character charAt(int i) {
				// TODO Auto-generated method stub
				return null;
			} 
	    } 
 
	public static void main(String[] args)throws IOException  {
		FastReader s=new FastReader();		
		int n =  s.nextInt();
		int[] ar = new int[n];
		double sum=0;
		for(int i=0;i<n;i++) {
			ar[i]=s.nextInt();
			sum+=(double)1/ar[i];
		}
		System.out.println(1/sum);
		
		
	}
	
}
