
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner jk = new Scanner(System.in);
		double n = jk.nextDouble();
		double[] arr = new double[(int) n];
		double den = 0.0;
		for (int i=0; i<n; i++)
		{
			arr[i] = jk.nextDouble();
			den+=(1/arr[i]);
		}
		System.out.println(1/den);
		
		
	}

} import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner jk = new Scanner(System.in);
		double n = jk.nextDouble();
		double[] arr = new double[(int) n];
		double den = 0.0;
		for (int i=0; i<n; i++)
		{
			arr[i] = jk.nextDouble();
			den+=(1/arr[i]);
		}
		System.out.println(1/den);
		
		
	}

} Main public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner jk = new Scanner(System.in);
		double n = jk.nextDouble();
		double[] arr = new double[(int) n];
		double den = 0.0;
		for (int i=0; i<n; i++)
		{
			arr[i] = jk.nextDouble();
			den+=(1/arr[i]);
		}
		System.out.println(1/den);
		
		
	} main {
		// TODO Auto-generated method stub
		Scanner jk = new Scanner(System.in);
		double n = jk.nextDouble();
		double[] arr = new double[(int) n];
		double den = 0.0;
		for (int i=0; i<n; i++)
		{
			arr[i] = jk.nextDouble();
			den+=(1/arr[i]);
		}
		System.out.println(1/den);
		
		
	} Scanner jk = new Scanner(System.in); jk new Scanner(System.in) double n = jk.nextDouble(); n jk.nextDouble() jk.nextDouble jk double[] arr = new double[(int) n]; arr new double[(int) n] (int) n double den = 0.0; den 0.0 for (int i=0; i<n; i++)
		{
			arr[i] = jk.nextDouble();
			den+=(1/arr[i]);
		} int i=0; int i=0; i 0 i<n i n i++ i++ i {
			arr[i] = jk.nextDouble();
			den+=(1/arr[i]);
		} {
			arr[i] = jk.nextDouble();
			den+=(1/arr[i]);
		} arr[i] = jk.nextDouble() arr[i] arr i jk.nextDouble() jk.nextDouble jk den+=(1/arr[i]) den (1/arr[i]) 1 arr[i] arr i System.out.println(1/den) System.out.println System.out System System.out 1/den 1 den String[] args args public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner jk = new Scanner(System.in);
		double n = jk.nextDouble();
		double[] arr = new double[(int) n];
		double den = 0.0;
		for (int i=0; i<n; i++)
		{
			arr[i] = jk.nextDouble();
			den+=(1/arr[i]);
		}
		System.out.println(1/den);
		
		
	}

} public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner jk = new Scanner(System.in);
		double n = jk.nextDouble();
		double[] arr = new double[(int) n];
		double den = 0.0;
		for (int i=0; i<n; i++)
		{
			arr[i] = jk.nextDouble();
			den+=(1/arr[i]);
		}
		System.out.println(1/den);
		
		
	}

} Main

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner jk = new Scanner(System.in);
		double n = jk.nextDouble();
		double[] arr = new double[(int) n];
		double den = 0.0;
		for (int i=0; i<n; i++)
		{
			arr[i] = jk.nextDouble();
			den+=(1/arr[i]);
		}
		System.out.println(1/den);
		
		
	}

}
