import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner scan = new Scanner(System.in);
        int N = scan.nextInt();
        //String temp = scan.nextLine();
        double A = 0;
        for(int i = 0 ; i < N ; i++){
            A = A + 1 / scan.nextDouble();
        }
        System.out.println(1 / A);
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner scan = new Scanner(System.in);
        int N = scan.nextInt();
        //String temp = scan.nextLine();
        double A = 0;
        for(int i = 0 ; i < N ; i++){
            A = A + 1 / scan.nextDouble();
        }
        System.out.println(1 / A);
    }
} Main public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner scan = new Scanner(System.in);
        int N = scan.nextInt();
        //String temp = scan.nextLine();
        double A = 0;
        for(int i = 0 ; i < N ; i++){
            A = A + 1 / scan.nextDouble();
        }
        System.out.println(1 / A);
    } main {
        // Your code here!
        Scanner scan = new Scanner(System.in);
        int N = scan.nextInt();
        //String temp = scan.nextLine();
        double A = 0;
        for(int i = 0 ; i < N ; i++){
            A = A + 1 / scan.nextDouble();
        }
        System.out.println(1 / A);
    } Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int N = scan.nextInt(); N scan.nextInt() scan.nextInt scan double A = 0; A 0 for(int i = 0 ; i < N ; i++){
            A = A + 1 / scan.nextDouble();
        } int i = 0 ; int i = 0 ; i 0 i < N i N i++ i++ i {
            A = A + 1 / scan.nextDouble();
        } {
            A = A + 1 / scan.nextDouble();
        } A = A + 1 / scan.nextDouble() A A + 1 / scan.nextDouble() A 1 / scan.nextDouble() 1 scan.nextDouble() scan.nextDouble scan System.out.println(1 / A) System.out.println System.out System System.out 1 / A 1 A String[] args args public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner scan = new Scanner(System.in);
        int N = scan.nextInt();
        //String temp = scan.nextLine();
        double A = 0;
        for(int i = 0 ; i < N ; i++){
            A = A + 1 / scan.nextDouble();
        }
        System.out.println(1 / A);
    }
} public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner scan = new Scanner(System.in);
        int N = scan.nextInt();
        //String temp = scan.nextLine();
        double A = 0;
        for(int i = 0 ; i < N ; i++){
            A = A + 1 / scan.nextDouble();
        }
        System.out.println(1 / A);
    }
} Main
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner scan = new Scanner(System.in);
        int N = scan.nextInt();
        //String temp = scan.nextLine();
        double A = 0;
        for(int i = 0 ; i < N ; i++){
            A = A + 1 / scan.nextDouble();
        }
        System.out.println(1 / A);
    }
}
