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


public class Main {
    static Scanner sc = new Scanner(System.in);
    public static void main(String args[]) {
        int n = sc.nextInt();
        double a = 1;
        double b = 0;
        for(int i = 0; i < n; i++) {
            int k = sc.nextInt();
           b += 1.0 / k;
        }
        

        System.out.println(a / b);


    } 
} import java.util.*; util.* java import java.math.*; math.* java import java.io.*; io.* java public class Main {
    static Scanner sc = new Scanner(System.in);
    public static void main(String args[]) {
        int n = sc.nextInt();
        double a = 1;
        double b = 0;
        for(int i = 0; i < n; i++) {
            int k = sc.nextInt();
           b += 1.0 / k;
        }
        

        System.out.println(a / b);


    } 
} Main static Scanner sc = new Scanner(System.in); sc new Scanner(System.in) public static void main(String args[]) {
        int n = sc.nextInt();
        double a = 1;
        double b = 0;
        for(int i = 0; i < n; i++) {
            int k = sc.nextInt();
           b += 1.0 / k;
        }
        

        System.out.println(a / b);


    } main {
        int n = sc.nextInt();
        double a = 1;
        double b = 0;
        for(int i = 0; i < n; i++) {
            int k = sc.nextInt();
           b += 1.0 / k;
        }
        

        System.out.println(a / b);


    } int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc double a = 1; a 1 double b = 0; b 0 for(int i = 0; i < n; i++) {
            int k = sc.nextInt();
           b += 1.0 / k;
        } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
            int k = sc.nextInt();
           b += 1.0 / k;
        } {
            int k = sc.nextInt();
           b += 1.0 / k;
        } int k = sc.nextInt(); k sc.nextInt() sc.nextInt sc b += 1.0 / k b 1.0 / k 1.0 k System.out.println(a / b) System.out.println System.out System System.out a / b a b String args[] args public class Main {
    static Scanner sc = new Scanner(System.in);
    public static void main(String args[]) {
        int n = sc.nextInt();
        double a = 1;
        double b = 0;
        for(int i = 0; i < n; i++) {
            int k = sc.nextInt();
           b += 1.0 / k;
        }
        

        System.out.println(a / b);


    } 
} public class Main {
    static Scanner sc = new Scanner(System.in);
    public static void main(String args[]) {
        int n = sc.nextInt();
        double a = 1;
        double b = 0;
        for(int i = 0; i < n; i++) {
            int k = sc.nextInt();
           b += 1.0 / k;
        }
        

        System.out.println(a / b);


    } 
} Main
import java.util.*;
import java.math.*;
import java.io.*;


public class Main {
    static Scanner sc = new Scanner(System.in);
    public static void main(String args[]) {
        int n = sc.nextInt();
        double a = 1;
        double b = 0;
        for(int i = 0; i < n; i++) {
            int k = sc.nextInt();
           b += 1.0 / k;
        }
        

        System.out.println(a / b);


    } 
}