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

public class Main
{
    PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

    public void go() throws IOException
    {
        StringTokenizer tok = new StringTokenizer(in.readLine());
        int n = Integer.parseInt(tok.nextToken());
        tok = new StringTokenizer(in.readLine());
        double sum = 0;
        for (int i = 0; i < n; i++)
        {
            double d = Integer.parseInt(tok.nextToken());
            d = 1/d;
            sum += d;
        }
        out.println(1 / sum);

        out.flush();
        in.close();
    }

    public static void main(String[] args) throws IOException
    {
        new Main().go();
    }
} import java.io.*; io.* java import java.util.*; util.* java public class Main
{
    PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

    public void go() throws IOException
    {
        StringTokenizer tok = new StringTokenizer(in.readLine());
        int n = Integer.parseInt(tok.nextToken());
        tok = new StringTokenizer(in.readLine());
        double sum = 0;
        for (int i = 0; i < n; i++)
        {
            double d = Integer.parseInt(tok.nextToken());
            d = 1/d;
            sum += d;
        }
        out.println(1 / sum);

        out.flush();
        in.close();
    }

    public static void main(String[] args) throws IOException
    {
        new Main().go();
    }
} Main PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out))); out new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out))) BufferedReader in = new BufferedReader(new InputStreamReader(System.in)); in new BufferedReader(new InputStreamReader(System.in)) public void go() throws IOException
    {
        StringTokenizer tok = new StringTokenizer(in.readLine());
        int n = Integer.parseInt(tok.nextToken());
        tok = new StringTokenizer(in.readLine());
        double sum = 0;
        for (int i = 0; i < n; i++)
        {
            double d = Integer.parseInt(tok.nextToken());
            d = 1/d;
            sum += d;
        }
        out.println(1 / sum);

        out.flush();
        in.close();
    } go {
        StringTokenizer tok = new StringTokenizer(in.readLine());
        int n = Integer.parseInt(tok.nextToken());
        tok = new StringTokenizer(in.readLine());
        double sum = 0;
        for (int i = 0; i < n; i++)
        {
            double d = Integer.parseInt(tok.nextToken());
            d = 1/d;
            sum += d;
        }
        out.println(1 / sum);

        out.flush();
        in.close();
    } StringTokenizer tok = new StringTokenizer(in.readLine()); tok new StringTokenizer(in.readLine()) int n = Integer.parseInt(tok.nextToken()); n Integer.parseInt(tok.nextToken()) Integer.parseInt Integer tok.nextToken() tok.nextToken tok tok = new StringTokenizer(in.readLine()) tok new StringTokenizer(in.readLine()) double sum = 0; sum 0 for (int i = 0; i < n; i++)
        {
            double d = Integer.parseInt(tok.nextToken());
            d = 1/d;
            sum += d;
        } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
            double d = Integer.parseInt(tok.nextToken());
            d = 1/d;
            sum += d;
        } {
            double d = Integer.parseInt(tok.nextToken());
            d = 1/d;
            sum += d;
        } double d = Integer.parseInt(tok.nextToken()); d Integer.parseInt(tok.nextToken()) Integer.parseInt Integer tok.nextToken() tok.nextToken tok d = 1/d d 1/d 1 d sum += d sum d out.println(1 / sum) out.println out 1 / sum 1 sum out.flush() out.flush out in.close() in.close in public static void main(String[] args) throws IOException
    {
        new Main().go();
    } main {
        new Main().go();
    } new Main().go() new Main().go new Main() String[] args args public class Main
{
    PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

    public void go() throws IOException
    {
        StringTokenizer tok = new StringTokenizer(in.readLine());
        int n = Integer.parseInt(tok.nextToken());
        tok = new StringTokenizer(in.readLine());
        double sum = 0;
        for (int i = 0; i < n; i++)
        {
            double d = Integer.parseInt(tok.nextToken());
            d = 1/d;
            sum += d;
        }
        out.println(1 / sum);

        out.flush();
        in.close();
    }

    public static void main(String[] args) throws IOException
    {
        new Main().go();
    }
} public class Main
{
    PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

    public void go() throws IOException
    {
        StringTokenizer tok = new StringTokenizer(in.readLine());
        int n = Integer.parseInt(tok.nextToken());
        tok = new StringTokenizer(in.readLine());
        double sum = 0;
        for (int i = 0; i < n; i++)
        {
            double d = Integer.parseInt(tok.nextToken());
            d = 1/d;
            sum += d;
        }
        out.println(1 / sum);

        out.flush();
        in.close();
    }

    public static void main(String[] args) throws IOException
    {
        new Main().go();
    }
} Main
import java.io.*;
import java.util.*;

public class Main
{
    PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

    public void go() throws IOException
    {
        StringTokenizer tok = new StringTokenizer(in.readLine());
        int n = Integer.parseInt(tok.nextToken());
        tok = new StringTokenizer(in.readLine());
        double sum = 0;
        for (int i = 0; i < n; i++)
        {
            double d = Integer.parseInt(tok.nextToken());
            d = 1/d;
            sum += d;
        }
        out.println(1 / sum);

        out.flush();
        in.close();
    }

    public static void main(String[] args) throws IOException
    {
        new Main().go();
    }
}
