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

public class Main {
    public static void main(String[] args) {
        Kattio in = new Kattio(System.in);
        int n = in.nextInt();
        
        double sum = 0;
        for (int i = 0; i < n; ++i) {
            double x = in.nextInt() * 1.0;
            sum += 1/x;
        }

        in.println(1 / sum);

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


}


class Kattio extends PrintWriter {
    public Kattio(InputStream i) {
	super(new BufferedOutputStream(System.out));
	r = new BufferedReader(new InputStreamReader(i));
    }
    public Kattio(InputStream i, OutputStream o) {
	super(new BufferedOutputStream(o));
	r = new BufferedReader(new InputStreamReader(i));
    }

    public boolean hasMoreTokens() {
	return peekToken() != null;
    }

    public int getInt() {
	return Integer.parseInt(nextToken());
    }

    public double getDouble() { 
	return Double.parseDouble(nextToken());
    }

    public long getLong() {
	return Long.parseLong(nextToken());
    }

    public String getWord() {
	return nextToken();
    }



    private BufferedReader r;
    private String line;
    private StringTokenizer st;
    private String token;

    private String peekToken() {
	if (token == null) 
	    try {
		while (st == null || !st.hasMoreTokens()) {
		    line = r.readLine();
		    if (line == null) return null;
		    st = new StringTokenizer(line);
		}
		token = st.nextToken();
	    } catch (IOException e) { }
	return token;
    }

    private String nextToken() {
        String ans = peekToken();
        token = null;
        return ans;
    }

    public int nextInt() {
        return Integer.parseInt(getWord());
    }

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

        in.println(1 / sum);

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


} Main public static void main(String[] args) {
        Kattio in = new Kattio(System.in);
        int n = in.nextInt();
        
        double sum = 0;
        for (int i = 0; i < n; ++i) {
            double x = in.nextInt() * 1.0;
            sum += 1/x;
        }

        in.println(1 / sum);

        in.close();
        in.flush();
    } main {
        Kattio in = new Kattio(System.in);
        int n = in.nextInt();
        
        double sum = 0;
        for (int i = 0; i < n; ++i) {
            double x = in.nextInt() * 1.0;
            sum += 1/x;
        }

        in.println(1 / sum);

        in.close();
        in.flush();
    } Kattio in = new Kattio(System.in); in new Kattio(System.in) int n = in.nextInt(); n in.nextInt() in.nextInt in double sum = 0; sum 0 for (int i = 0; i < n; ++i) {
            double x = in.nextInt() * 1.0;
            sum += 1/x;
        } int i = 0; int i = 0; i 0 i < n i n ++i ++i i {
            double x = in.nextInt() * 1.0;
            sum += 1/x;
        } {
            double x = in.nextInt() * 1.0;
            sum += 1/x;
        } double x = in.nextInt() * 1.0; x in.nextInt() * 1.0 in.nextInt() in.nextInt in 1.0 sum += 1/x sum 1/x 1 x in.println(1 / sum) in.println in 1 / sum 1 sum in.close() in.close in in.flush() in.flush in String[] args args class Kattio extends PrintWriter {
    public Kattio(InputStream i) {
	super(new BufferedOutputStream(System.out));
	r = new BufferedReader(new InputStreamReader(i));
    }
    public Kattio(InputStream i, OutputStream o) {
	super(new BufferedOutputStream(o));
	r = new BufferedReader(new InputStreamReader(i));
    }

    public boolean hasMoreTokens() {
	return peekToken() != null;
    }

    public int getInt() {
	return Integer.parseInt(nextToken());
    }

    public double getDouble() { 
	return Double.parseDouble(nextToken());
    }

    public long getLong() {
	return Long.parseLong(nextToken());
    }

    public String getWord() {
	return nextToken();
    }



    private BufferedReader r;
    private String line;
    private StringTokenizer st;
    private String token;

    private String peekToken() {
	if (token == null) 
	    try {
		while (st == null || !st.hasMoreTokens()) {
		    line = r.readLine();
		    if (line == null) return null;
		    st = new StringTokenizer(line);
		}
		token = st.nextToken();
	    } catch (IOException e) { }
	return token;
    }

    private String nextToken() {
        String ans = peekToken();
        token = null;
        return ans;
    }

    public int nextInt() {
        return Integer.parseInt(getWord());
    }

} Kattio private BufferedReader r; r private String line; line private StringTokenizer st; st private String token; token public Kattio(InputStream i) {
	super(new BufferedOutputStream(System.out));
	r = new BufferedReader(new InputStreamReader(i));
    } Kattio {
	super(new BufferedOutputStream(System.out));
	r = new BufferedReader(new InputStreamReader(i));
    } super(new BufferedOutputStream(System.out)) super new BufferedOutputStream(System.out) r = new BufferedReader(new InputStreamReader(i)) r new BufferedReader(new InputStreamReader(i)) InputStream i i public Kattio(InputStream i, OutputStream o) {
	super(new BufferedOutputStream(o));
	r = new BufferedReader(new InputStreamReader(i));
    } Kattio {
	super(new BufferedOutputStream(o));
	r = new BufferedReader(new InputStreamReader(i));
    } super(new BufferedOutputStream(o)) super new BufferedOutputStream(o) r = new BufferedReader(new InputStreamReader(i)) r new BufferedReader(new InputStreamReader(i)) InputStream i i OutputStream o o public boolean hasMoreTokens() {
	return peekToken() != null;
    } hasMoreTokens {
	return peekToken() != null;
    } return peekToken() != null; peekToken() != null peekToken() peekToken null public int getInt() {
	return Integer.parseInt(nextToken());
    } getInt {
	return Integer.parseInt(nextToken());
    } return Integer.parseInt(nextToken()); Integer.parseInt(nextToken()) Integer.parseInt Integer nextToken() nextToken public double getDouble() { 
	return Double.parseDouble(nextToken());
    } getDouble { 
	return Double.parseDouble(nextToken());
    } return Double.parseDouble(nextToken()); Double.parseDouble(nextToken()) Double.parseDouble Double nextToken() nextToken public long getLong() {
	return Long.parseLong(nextToken());
    } getLong {
	return Long.parseLong(nextToken());
    } return Long.parseLong(nextToken()); Long.parseLong(nextToken()) Long.parseLong Long nextToken() nextToken public String getWord() {
	return nextToken();
    } getWord {
	return nextToken();
    } return nextToken(); nextToken() nextToken private String peekToken() {
	if (token == null) 
	    try {
		while (st == null || !st.hasMoreTokens()) {
		    line = r.readLine();
		    if (line == null) return null;
		    st = new StringTokenizer(line);
		}
		token = st.nextToken();
	    } catch (IOException e) { }
	return token;
    } peekToken {
	if (token == null) 
	    try {
		while (st == null || !st.hasMoreTokens()) {
		    line = r.readLine();
		    if (line == null) return null;
		    st = new StringTokenizer(line);
		}
		token = st.nextToken();
	    } catch (IOException e) { }
	return token;
    } if (token == null) 
	    try {
		while (st == null || !st.hasMoreTokens()) {
		    line = r.readLine();
		    if (line == null) return null;
		    st = new StringTokenizer(line);
		}
		token = st.nextToken();
	    } catch (IOException e) { } token == null token null try {
		while (st == null || !st.hasMoreTokens()) {
		    line = r.readLine();
		    if (line == null) return null;
		    st = new StringTokenizer(line);
		}
		token = st.nextToken();
	    } catch (IOException e) { } catch (IOException e) { } IOException e { } {
		while (st == null || !st.hasMoreTokens()) {
		    line = r.readLine();
		    if (line == null) return null;
		    st = new StringTokenizer(line);
		}
		token = st.nextToken();
	    } while (st == null || !st.hasMoreTokens()) {
		    line = r.readLine();
		    if (line == null) return null;
		    st = new StringTokenizer(line);
		} st == null || !st.hasMoreTokens() st == null st null !st.hasMoreTokens() st.hasMoreTokens() st.hasMoreTokens st {
		    line = r.readLine();
		    if (line == null) return null;
		    st = new StringTokenizer(line);
		} line = r.readLine() line r.readLine() r.readLine r if (line == null) return null; line == null line null return null; null st = new StringTokenizer(line) st new StringTokenizer(line) token = st.nextToken() token st.nextToken() st.nextToken st return token; token private String nextToken() {
        String ans = peekToken();
        token = null;
        return ans;
    } nextToken {
        String ans = peekToken();
        token = null;
        return ans;
    } String ans = peekToken(); ans peekToken() peekToken token = null token null return ans; ans public int nextInt() {
        return Integer.parseInt(getWord());
    } nextInt {
        return Integer.parseInt(getWord());
    } return Integer.parseInt(getWord()); Integer.parseInt(getWord()) Integer.parseInt Integer getWord() getWord PrintWriter public class Main {
    public static void main(String[] args) {
        Kattio in = new Kattio(System.in);
        int n = in.nextInt();
        
        double sum = 0;
        for (int i = 0; i < n; ++i) {
            double x = in.nextInt() * 1.0;
            sum += 1/x;
        }

        in.println(1 / sum);

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


} public class Main {
    public static void main(String[] args) {
        Kattio in = new Kattio(System.in);
        int n = in.nextInt();
        
        double sum = 0;
        for (int i = 0; i < n; ++i) {
            double x = in.nextInt() * 1.0;
            sum += 1/x;
        }

        in.println(1 / sum);

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


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

public class Main {
    public static void main(String[] args) {
        Kattio in = new Kattio(System.in);
        int n = in.nextInt();
        
        double sum = 0;
        for (int i = 0; i < n; ++i) {
            double x = in.nextInt() * 1.0;
            sum += 1/x;
        }

        in.println(1 / sum);

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


}


class Kattio extends PrintWriter {
    public Kattio(InputStream i) {
	super(new BufferedOutputStream(System.out));
	r = new BufferedReader(new InputStreamReader(i));
    }
    public Kattio(InputStream i, OutputStream o) {
	super(new BufferedOutputStream(o));
	r = new BufferedReader(new InputStreamReader(i));
    }

    public boolean hasMoreTokens() {
	return peekToken() != null;
    }

    public int getInt() {
	return Integer.parseInt(nextToken());
    }

    public double getDouble() { 
	return Double.parseDouble(nextToken());
    }

    public long getLong() {
	return Long.parseLong(nextToken());
    }

    public String getWord() {
	return nextToken();
    }



    private BufferedReader r;
    private String line;
    private StringTokenizer st;
    private String token;

    private String peekToken() {
	if (token == null) 
	    try {
		while (st == null || !st.hasMoreTokens()) {
		    line = r.readLine();
		    if (line == null) return null;
		    st = new StringTokenizer(line);
		}
		token = st.nextToken();
	    } catch (IOException e) { }
	return token;
    }

    private String nextToken() {
        String ans = peekToken();
        token = null;
        return ans;
    }

    public int nextInt() {
        return Integer.parseInt(getWord());
    }

}