import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.PrintWriter;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) throws Exception {
        try (BufferedInputStream in = new BufferedInputStream(System.in);
             PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out))) {

            Scanner sc = new Scanner(in);

            // 11:46-

            // 11:46-

            int n = sc.nextInt();
            double sum = 0.0;
            for (int i = 0; i < n; i++) sum += 1 / sc.nextDouble();

            out.println(1 / sum);

        }
    }

} import java.io.BufferedInputStream; BufferedInputStream java.io import java.io.BufferedOutputStream; BufferedOutputStream java.io import java.io.PrintWriter; PrintWriter java.io import java.util.Scanner; Scanner java.util public class Main {

    public static void main(String[] args) throws Exception {
        try (BufferedInputStream in = new BufferedInputStream(System.in);
             PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out))) {

            Scanner sc = new Scanner(in);

            // 11:46-

            // 11:46-

            int n = sc.nextInt();
            double sum = 0.0;
            for (int i = 0; i < n; i++) sum += 1 / sc.nextDouble();

            out.println(1 / sum);

        }
    }

} Main public static void main(String[] args) throws Exception {
        try (BufferedInputStream in = new BufferedInputStream(System.in);
             PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out))) {

            Scanner sc = new Scanner(in);

            // 11:46-

            // 11:46-

            int n = sc.nextInt();
            double sum = 0.0;
            for (int i = 0; i < n; i++) sum += 1 / sc.nextDouble();

            out.println(1 / sum);

        }
    } main {
        try (BufferedInputStream in = new BufferedInputStream(System.in);
             PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out))) {

            Scanner sc = new Scanner(in);

            // 11:46-

            // 11:46-

            int n = sc.nextInt();
            double sum = 0.0;
            for (int i = 0; i < n; i++) sum += 1 / sc.nextDouble();

            out.println(1 / sum);

        }
    } try (BufferedInputStream in = new BufferedInputStream(System.in);
             PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out))) {

            Scanner sc = new Scanner(in);

            // 11:46-

            // 11:46-

            int n = sc.nextInt();
            double sum = 0.0;
            for (int i = 0; i < n; i++) sum += 1 / sc.nextDouble();

            out.println(1 / sum);

        } {

            Scanner sc = new Scanner(in);

            // 11:46-

            // 11:46-

            int n = sc.nextInt();
            double sum = 0.0;
            for (int i = 0; i < n; i++) sum += 1 / sc.nextDouble();

            out.println(1 / sum);

        } BufferedInputStream in = new BufferedInputStream(System.in) BufferedInputStream in = new BufferedInputStream(System.in) new BufferedInputStream(System.in) PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out)) PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out)) new PrintWriter(new BufferedOutputStream(System.out)) Scanner sc = new Scanner(in); sc new Scanner(in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc double sum = 0.0; sum 0.0 for (int i = 0; i < n; i++) sum += 1 / sc.nextDouble(); int i = 0; int i = 0; i 0 i < n i n i++ i++ i sum += 1 / sc.nextDouble(); sum += 1 / sc.nextDouble() sum 1 / sc.nextDouble() 1 sc.nextDouble() sc.nextDouble sc out.println(1 / sum) out.println out 1 / sum 1 sum String[] args args public class Main {

    public static void main(String[] args) throws Exception {
        try (BufferedInputStream in = new BufferedInputStream(System.in);
             PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out))) {

            Scanner sc = new Scanner(in);

            // 11:46-

            // 11:46-

            int n = sc.nextInt();
            double sum = 0.0;
            for (int i = 0; i < n; i++) sum += 1 / sc.nextDouble();

            out.println(1 / sum);

        }
    }

} public class Main {

    public static void main(String[] args) throws Exception {
        try (BufferedInputStream in = new BufferedInputStream(System.in);
             PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out))) {

            Scanner sc = new Scanner(in);

            // 11:46-

            // 11:46-

            int n = sc.nextInt();
            double sum = 0.0;
            for (int i = 0; i < n; i++) sum += 1 / sc.nextDouble();

            out.println(1 / sum);

        }
    }

} Main
import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.PrintWriter;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) throws Exception {
        try (BufferedInputStream in = new BufferedInputStream(System.in);
             PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out))) {

            Scanner sc = new Scanner(in);

            // 11:46-

            // 11:46-

            int n = sc.nextInt();
            double sum = 0.0;
            for (int i = 0; i < n; i++) sum += 1 / sc.nextDouble();

            out.println(1 / sum);

        }
    }

}
