import java.awt.Polygon;
import java.awt.geom.Area;
import java.awt.geom.PathIterator;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Scanner;

public class Main {
	
	public static void main(String[] args) throws Exception
	{
		new Main().run();
	}
	
	public void run() throws Exception
	{
		Scanner file = new Scanner(System.in);
		int N = file.nextInt();
		double[] inv = new double[N];
		for(int i = 0;i<N;i++)
		{
			inv[i] = 1/file.nextDouble();
		}
		double sum = 0;
		for(double x:inv)
			sum+=x;
		System.out.println(1/sum);
	}
} import java.awt.Polygon; Polygon java.awt import java.awt.geom.Area; Area java.awt.geom import java.awt.geom.PathIterator; PathIterator java.awt.geom import java.util.ArrayList; ArrayList java.util import java.util.Arrays; Arrays java.util import java.util.Scanner; Scanner java.util public class Main {
	
	public static void main(String[] args) throws Exception
	{
		new Main().run();
	}
	
	public void run() throws Exception
	{
		Scanner file = new Scanner(System.in);
		int N = file.nextInt();
		double[] inv = new double[N];
		for(int i = 0;i<N;i++)
		{
			inv[i] = 1/file.nextDouble();
		}
		double sum = 0;
		for(double x:inv)
			sum+=x;
		System.out.println(1/sum);
	}
} Main public static void main(String[] args) throws Exception
	{
		new Main().run();
	} main {
		new Main().run();
	} new Main().run() new Main().run new Main() String[] args args public void run() throws Exception
	{
		Scanner file = new Scanner(System.in);
		int N = file.nextInt();
		double[] inv = new double[N];
		for(int i = 0;i<N;i++)
		{
			inv[i] = 1/file.nextDouble();
		}
		double sum = 0;
		for(double x:inv)
			sum+=x;
		System.out.println(1/sum);
	} run {
		Scanner file = new Scanner(System.in);
		int N = file.nextInt();
		double[] inv = new double[N];
		for(int i = 0;i<N;i++)
		{
			inv[i] = 1/file.nextDouble();
		}
		double sum = 0;
		for(double x:inv)
			sum+=x;
		System.out.println(1/sum);
	} Scanner file = new Scanner(System.in); file new Scanner(System.in) int N = file.nextInt(); N file.nextInt() file.nextInt file double[] inv = new double[N]; inv new double[N] N for(int i = 0;i<N;i++)
		{
			inv[i] = 1/file.nextDouble();
		} int i = 0; int i = 0; i 0 i<N i N i++ i++ i {
			inv[i] = 1/file.nextDouble();
		} {
			inv[i] = 1/file.nextDouble();
		} inv[i] = 1/file.nextDouble() inv[i] inv i 1/file.nextDouble() 1 file.nextDouble() file.nextDouble file double sum = 0; sum 0 for(double x:inv)
			sum+=x; double x inv sum+=x; sum+=x sum x System.out.println(1/sum) System.out.println System.out System System.out 1/sum 1 sum public class Main {
	
	public static void main(String[] args) throws Exception
	{
		new Main().run();
	}
	
	public void run() throws Exception
	{
		Scanner file = new Scanner(System.in);
		int N = file.nextInt();
		double[] inv = new double[N];
		for(int i = 0;i<N;i++)
		{
			inv[i] = 1/file.nextDouble();
		}
		double sum = 0;
		for(double x:inv)
			sum+=x;
		System.out.println(1/sum);
	}
} public class Main {
	
	public static void main(String[] args) throws Exception
	{
		new Main().run();
	}
	
	public void run() throws Exception
	{
		Scanner file = new Scanner(System.in);
		int N = file.nextInt();
		double[] inv = new double[N];
		for(int i = 0;i<N;i++)
		{
			inv[i] = 1/file.nextDouble();
		}
		double sum = 0;
		for(double x:inv)
			sum+=x;
		System.out.println(1/sum);
	}
} Main
import java.awt.Polygon;
import java.awt.geom.Area;
import java.awt.geom.PathIterator;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Scanner;

public class Main {
	
	public static void main(String[] args) throws Exception
	{
		new Main().run();
	}
	
	public void run() throws Exception
	{
		Scanner file = new Scanner(System.in);
		int N = file.nextInt();
		double[] inv = new double[N];
		for(int i = 0;i<N;i++)
		{
			inv[i] = 1/file.nextDouble();
		}
		double sum = 0;
		for(double x:inv)
			sum+=x;
		System.out.println(1/sum);
	}
}
