import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
	
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        OutputStream outputStream = System.out;
    	PrintWriter out = new PrintWriter(outputStream);

        int i, n, m, c, k;

		int t = Integer.parseInt(br.readLine());    
        StringTokenizer st = new StringTokenizer(br.readLine());
    
      	double sum=0;
		for(i=0;i<t;i++){
          double ar=Double.parseDouble(st.nextToken());
          sum+=1/ar;
          
        }
      	
      	//sum=power(sum,mod-2)%mod;
      out.print(1/sum);
	out.flush();
	out.close();
	
	}
} import java.io.*; io.* java import java.util.*; util.* java public class Main {
    public static void main(String[] args) throws Exception {
	
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        OutputStream outputStream = System.out;
    	PrintWriter out = new PrintWriter(outputStream);

        int i, n, m, c, k;

		int t = Integer.parseInt(br.readLine());    
        StringTokenizer st = new StringTokenizer(br.readLine());
    
      	double sum=0;
		for(i=0;i<t;i++){
          double ar=Double.parseDouble(st.nextToken());
          sum+=1/ar;
          
        }
      	
      	//sum=power(sum,mod-2)%mod;
      out.print(1/sum);
	out.flush();
	out.close();
	
	}
} Main public static void main(String[] args) throws Exception {
	
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        OutputStream outputStream = System.out;
    	PrintWriter out = new PrintWriter(outputStream);

        int i, n, m, c, k;

		int t = Integer.parseInt(br.readLine());    
        StringTokenizer st = new StringTokenizer(br.readLine());
    
      	double sum=0;
		for(i=0;i<t;i++){
          double ar=Double.parseDouble(st.nextToken());
          sum+=1/ar;
          
        }
      	
      	//sum=power(sum,mod-2)%mod;
      out.print(1/sum);
	out.flush();
	out.close();
	
	} main {
	
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        OutputStream outputStream = System.out;
    	PrintWriter out = new PrintWriter(outputStream);

        int i, n, m, c, k;

		int t = Integer.parseInt(br.readLine());    
        StringTokenizer st = new StringTokenizer(br.readLine());
    
      	double sum=0;
		for(i=0;i<t;i++){
          double ar=Double.parseDouble(st.nextToken());
          sum+=1/ar;
          
        }
      	
      	//sum=power(sum,mod-2)%mod;
      out.print(1/sum);
	out.flush();
	out.close();
	
	} BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); br new BufferedReader(new InputStreamReader(System.in)) OutputStream outputStream = System.out; outputStream System.out System System.out PrintWriter out = new PrintWriter(outputStream); out new PrintWriter(outputStream) int i i n n m m c c k; k int t = Integer.parseInt(br.readLine()); t Integer.parseInt(br.readLine()) Integer.parseInt Integer br.readLine() br.readLine br StringTokenizer st = new StringTokenizer(br.readLine()); st new StringTokenizer(br.readLine()) double sum=0; sum 0 for(i=0;i<t;i++){
          double ar=Double.parseDouble(st.nextToken());
          sum+=1/ar;
          
        } i=0; i=0 i 0 i<t i t i++ i++ i {
          double ar=Double.parseDouble(st.nextToken());
          sum+=1/ar;
          
        } {
          double ar=Double.parseDouble(st.nextToken());
          sum+=1/ar;
          
        } double ar=Double.parseDouble(st.nextToken()); ar Double.parseDouble(st.nextToken()) Double.parseDouble Double st.nextToken() st.nextToken st sum+=1/ar sum 1/ar 1 ar out.print(1/sum) out.print out 1/sum 1 sum out.flush() out.flush out out.close() out.close out String[] args args public class Main {
    public static void main(String[] args) throws Exception {
	
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        OutputStream outputStream = System.out;
    	PrintWriter out = new PrintWriter(outputStream);

        int i, n, m, c, k;

		int t = Integer.parseInt(br.readLine());    
        StringTokenizer st = new StringTokenizer(br.readLine());
    
      	double sum=0;
		for(i=0;i<t;i++){
          double ar=Double.parseDouble(st.nextToken());
          sum+=1/ar;
          
        }
      	
      	//sum=power(sum,mod-2)%mod;
      out.print(1/sum);
	out.flush();
	out.close();
	
	}
} public class Main {
    public static void main(String[] args) throws Exception {
	
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        OutputStream outputStream = System.out;
    	PrintWriter out = new PrintWriter(outputStream);

        int i, n, m, c, k;

		int t = Integer.parseInt(br.readLine());    
        StringTokenizer st = new StringTokenizer(br.readLine());
    
      	double sum=0;
		for(i=0;i<t;i++){
          double ar=Double.parseDouble(st.nextToken());
          sum+=1/ar;
          
        }
      	
      	//sum=power(sum,mod-2)%mod;
      out.print(1/sum);
	out.flush();
	out.close();
	
	}
} Main
import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
	
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        OutputStream outputStream = System.out;
    	PrintWriter out = new PrintWriter(outputStream);

        int i, n, m, c, k;

		int t = Integer.parseInt(br.readLine());    
        StringTokenizer st = new StringTokenizer(br.readLine());
    
      	double sum=0;
		for(i=0;i<t;i++){
          double ar=Double.parseDouble(st.nextToken());
          sum+=1/ar;
          
        }
      	
      	//sum=power(sum,mod-2)%mod;
      out.print(1/sum);
	out.flush();
	out.close();
	
	}
}