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

class Main {

    public static void main(String[] args) {
        MyScanner sc = new MyScanner();
        
        int n = sc.nextInt();

        double s=0;

        int[] A = new int[100];
        for(int i=0;i<n;i++){
            s += 1.0/sc.nextInt();
        }

        System.out.println(1/s);

    }

    static class MyScanner {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in), 1 << 15);
        StringTokenizer tokenizer;
    
        String next() {
            try {
                while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                    tokenizer = new StringTokenizer(reader.readLine());
                }
            } catch (IOException ignored) {
            }
            return tokenizer.nextToken();
        }
    
        int nextInt() {
            return Integer.parseInt(next());
        }
    
        long nextLong() {
            return Long.parseLong(next());
        }
    }
} import java.io.*; io.* java import java.util.*; util.* java class Main {

    public static void main(String[] args) {
        MyScanner sc = new MyScanner();
        
        int n = sc.nextInt();

        double s=0;

        int[] A = new int[100];
        for(int i=0;i<n;i++){
            s += 1.0/sc.nextInt();
        }

        System.out.println(1/s);

    }

    static class MyScanner {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in), 1 << 15);
        StringTokenizer tokenizer;
    
        String next() {
            try {
                while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                    tokenizer = new StringTokenizer(reader.readLine());
                }
            } catch (IOException ignored) {
            }
            return tokenizer.nextToken();
        }
    
        int nextInt() {
            return Integer.parseInt(next());
        }
    
        long nextLong() {
            return Long.parseLong(next());
        }
    }
} Main public static void main(String[] args) {
        MyScanner sc = new MyScanner();
        
        int n = sc.nextInt();

        double s=0;

        int[] A = new int[100];
        for(int i=0;i<n;i++){
            s += 1.0/sc.nextInt();
        }

        System.out.println(1/s);

    } main {
        MyScanner sc = new MyScanner();
        
        int n = sc.nextInt();

        double s=0;

        int[] A = new int[100];
        for(int i=0;i<n;i++){
            s += 1.0/sc.nextInt();
        }

        System.out.println(1/s);

    } MyScanner sc = new MyScanner(); sc new MyScanner() int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc double s=0; s 0 int[] A = new int[100]; A new int[100] 100 for(int i=0;i<n;i++){
            s += 1.0/sc.nextInt();
        } int i=0; int i=0; i 0 i<n i n i++ i++ i {
            s += 1.0/sc.nextInt();
        } {
            s += 1.0/sc.nextInt();
        } s += 1.0/sc.nextInt() s 1.0/sc.nextInt() 1.0 sc.nextInt() sc.nextInt sc System.out.println(1/s) System.out.println System.out System System.out 1/s 1 s String[] args args static class MyScanner {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in), 1 << 15);
        StringTokenizer tokenizer;
    
        String next() {
            try {
                while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                    tokenizer = new StringTokenizer(reader.readLine());
                }
            } catch (IOException ignored) {
            }
            return tokenizer.nextToken();
        }
    
        int nextInt() {
            return Integer.parseInt(next());
        }
    
        long nextLong() {
            return Long.parseLong(next());
        }
    } MyScanner BufferedReader reader = new BufferedReader(new InputStreamReader(System.in), 1 << 15); reader new BufferedReader(new InputStreamReader(System.in), 1 << 15) StringTokenizer tokenizer; tokenizer String next() {
            try {
                while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                    tokenizer = new StringTokenizer(reader.readLine());
                }
            } catch (IOException ignored) {
            }
            return tokenizer.nextToken();
        } next {
            try {
                while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                    tokenizer = new StringTokenizer(reader.readLine());
                }
            } catch (IOException ignored) {
            }
            return tokenizer.nextToken();
        } try {
                while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                    tokenizer = new StringTokenizer(reader.readLine());
                }
            } catch (IOException ignored) {
            } catch (IOException ignored) {
            } IOException ignored {
            } {
                while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                    tokenizer = new StringTokenizer(reader.readLine());
                }
            } while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                    tokenizer = new StringTokenizer(reader.readLine());
                } tokenizer == null || !tokenizer.hasMoreTokens() tokenizer == null tokenizer null !tokenizer.hasMoreTokens() tokenizer.hasMoreTokens() tokenizer.hasMoreTokens tokenizer {
                    tokenizer = new StringTokenizer(reader.readLine());
                } tokenizer = new StringTokenizer(reader.readLine()) tokenizer new StringTokenizer(reader.readLine()) return tokenizer.nextToken(); tokenizer.nextToken() tokenizer.nextToken tokenizer 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
import java.io.*;
import java.util.*;

class Main {

    public static void main(String[] args) {
        MyScanner sc = new MyScanner();
        
        int n = sc.nextInt();

        double s=0;

        int[] A = new int[100];
        for(int i=0;i<n;i++){
            s += 1.0/sc.nextInt();
        }

        System.out.println(1/s);

    }

    static class MyScanner {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in), 1 << 15);
        StringTokenizer tokenizer;
    
        String next() {
            try {
                while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                    tokenizer = new StringTokenizer(reader.readLine());
                }
            } catch (IOException ignored) {
            }
            return tokenizer.nextToken();
        }
    
        int nextInt() {
            return Integer.parseInt(next());
        }
    
        long nextLong() {
            return Long.parseLong(next());
        }
    }
}