import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        Scanner in = new Scanner(System.in);
        int n = Integer.parseInt(in.nextLine());
        String[] params = in.nextLine().split(" ");

        double parent = 0;
        for( int i = 0 ; i < n ; i++ ){
            parent += 1 / Double.parseDouble(params[i]);
        }

        System.out.println( 1/parent );

    }

} import java.util.Scanner; Scanner java.util public class Main {

    public static void main(String[] args) {

        Scanner in = new Scanner(System.in);
        int n = Integer.parseInt(in.nextLine());
        String[] params = in.nextLine().split(" ");

        double parent = 0;
        for( int i = 0 ; i < n ; i++ ){
            parent += 1 / Double.parseDouble(params[i]);
        }

        System.out.println( 1/parent );

    }

} Main public static void main(String[] args) {

        Scanner in = new Scanner(System.in);
        int n = Integer.parseInt(in.nextLine());
        String[] params = in.nextLine().split(" ");

        double parent = 0;
        for( int i = 0 ; i < n ; i++ ){
            parent += 1 / Double.parseDouble(params[i]);
        }

        System.out.println( 1/parent );

    } main {

        Scanner in = new Scanner(System.in);
        int n = Integer.parseInt(in.nextLine());
        String[] params = in.nextLine().split(" ");

        double parent = 0;
        for( int i = 0 ; i < n ; i++ ){
            parent += 1 / Double.parseDouble(params[i]);
        }

        System.out.println( 1/parent );

    } Scanner in = new Scanner(System.in); in new Scanner(System.in) int n = Integer.parseInt(in.nextLine()); n Integer.parseInt(in.nextLine()) Integer.parseInt Integer in.nextLine() in.nextLine in String[] params = in.nextLine().split(" "); params in.nextLine().split(" ") in.nextLine().split in.nextLine() in.nextLine in " " double parent = 0; parent 0 for( int i = 0 ; i < n ; i++ ){
            parent += 1 / Double.parseDouble(params[i]);
        } int i = 0 ; int i = 0 ; i 0 i < n i n i++ i++ i {
            parent += 1 / Double.parseDouble(params[i]);
        } {
            parent += 1 / Double.parseDouble(params[i]);
        } parent += 1 / Double.parseDouble(params[i]) parent 1 / Double.parseDouble(params[i]) 1 Double.parseDouble(params[i]) Double.parseDouble Double params[i] params i System.out.println( 1/parent ) System.out.println System.out System System.out 1/parent 1 parent String[] args args public class Main {

    public static void main(String[] args) {

        Scanner in = new Scanner(System.in);
        int n = Integer.parseInt(in.nextLine());
        String[] params = in.nextLine().split(" ");

        double parent = 0;
        for( int i = 0 ; i < n ; i++ ){
            parent += 1 / Double.parseDouble(params[i]);
        }

        System.out.println( 1/parent );

    }

} public class Main {

    public static void main(String[] args) {

        Scanner in = new Scanner(System.in);
        int n = Integer.parseInt(in.nextLine());
        String[] params = in.nextLine().split(" ");

        double parent = 0;
        for( int i = 0 ; i < n ; i++ ){
            parent += 1 / Double.parseDouble(params[i]);
        }

        System.out.println( 1/parent );

    }

} Main
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        Scanner in = new Scanner(System.in);
        int n = Integer.parseInt(in.nextLine());
        String[] params = in.nextLine().split(" ");

        double parent = 0;
        for( int i = 0 ; i < n ; i++ ){
            parent += 1 / Double.parseDouble(params[i]);
        }

        System.out.println( 1/parent );

    }

}
