import java.util.*;

public class Main
{
	public static void main(String [] Args)
	{
		Scanner in = new Scanner(System.in);
		int n=in.nextInt();
		double ans=0.0;
		for(int i=0;i<n;i++)
		{
			double q=in.nextDouble();
			ans=ans+1.0/q;
		}
		ans=1.0/ans;
		System.out.println(ans);
	}
}
	 import java.util.*; util.* java public class Main
{
	public static void main(String [] Args)
	{
		Scanner in = new Scanner(System.in);
		int n=in.nextInt();
		double ans=0.0;
		for(int i=0;i<n;i++)
		{
			double q=in.nextDouble();
			ans=ans+1.0/q;
		}
		ans=1.0/ans;
		System.out.println(ans);
	}
} Main public static void main(String [] Args)
	{
		Scanner in = new Scanner(System.in);
		int n=in.nextInt();
		double ans=0.0;
		for(int i=0;i<n;i++)
		{
			double q=in.nextDouble();
			ans=ans+1.0/q;
		}
		ans=1.0/ans;
		System.out.println(ans);
	} main {
		Scanner in = new Scanner(System.in);
		int n=in.nextInt();
		double ans=0.0;
		for(int i=0;i<n;i++)
		{
			double q=in.nextDouble();
			ans=ans+1.0/q;
		}
		ans=1.0/ans;
		System.out.println(ans);
	} Scanner in = new Scanner(System.in); in new Scanner(System.in) int n=in.nextInt(); n in.nextInt() in.nextInt in double ans=0.0; ans 0.0 for(int i=0;i<n;i++)
		{
			double q=in.nextDouble();
			ans=ans+1.0/q;
		} int i=0; int i=0; i 0 i<n i n i++ i++ i {
			double q=in.nextDouble();
			ans=ans+1.0/q;
		} {
			double q=in.nextDouble();
			ans=ans+1.0/q;
		} double q=in.nextDouble(); q in.nextDouble() in.nextDouble in ans=ans+1.0/q ans ans+1.0/q ans 1.0/q 1.0 q ans=1.0/ans ans 1.0/ans 1.0 ans System.out.println(ans) System.out.println System.out System System.out ans String [] Args Args public class Main
{
	public static void main(String [] Args)
	{
		Scanner in = new Scanner(System.in);
		int n=in.nextInt();
		double ans=0.0;
		for(int i=0;i<n;i++)
		{
			double q=in.nextDouble();
			ans=ans+1.0/q;
		}
		ans=1.0/ans;
		System.out.println(ans);
	}
} public class Main
{
	public static void main(String [] Args)
	{
		Scanner in = new Scanner(System.in);
		int n=in.nextInt();
		double ans=0.0;
		for(int i=0;i<n;i++)
		{
			double q=in.nextDouble();
			ans=ans+1.0/q;
		}
		ans=1.0/ans;
		System.out.println(ans);
	}
} Main
import java.util.*;

public class Main
{
	public static void main(String [] Args)
	{
		Scanner in = new Scanner(System.in);
		int n=in.nextInt();
		double ans=0.0;
		for(int i=0;i<n;i++)
		{
			double q=in.nextDouble();
			ans=ans+1.0/q;
		}
		ans=1.0/ans;
		System.out.println(ans);
	}
}
	