import java.util.Scanner;

public class Main{

	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int N = Integer.parseInt(sc.nextLine());
		int[] A = new int[N];
		double d = 0;
		for(int i = 0;i < N;i++){
			A[i] = Integer.parseInt(sc.next());
			d += 1/(double)A[i];
		}
		sc.close();
		System.out.println(1/d);
	}
} import java.util.Scanner; Scanner java.util public class Main{

	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int N = Integer.parseInt(sc.nextLine());
		int[] A = new int[N];
		double d = 0;
		for(int i = 0;i < N;i++){
			A[i] = Integer.parseInt(sc.next());
			d += 1/(double)A[i];
		}
		sc.close();
		System.out.println(1/d);
	}
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int N = Integer.parseInt(sc.nextLine());
		int[] A = new int[N];
		double d = 0;
		for(int i = 0;i < N;i++){
			A[i] = Integer.parseInt(sc.next());
			d += 1/(double)A[i];
		}
		sc.close();
		System.out.println(1/d);
	} main {
		Scanner sc = new Scanner(System.in);
		int N = Integer.parseInt(sc.nextLine());
		int[] A = new int[N];
		double d = 0;
		for(int i = 0;i < N;i++){
			A[i] = Integer.parseInt(sc.next());
			d += 1/(double)A[i];
		}
		sc.close();
		System.out.println(1/d);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = Integer.parseInt(sc.nextLine()); N Integer.parseInt(sc.nextLine()) Integer.parseInt Integer sc.nextLine() sc.nextLine sc int[] A = new int[N]; A new int[N] N double d = 0; d 0 for(int i = 0;i < N;i++){
			A[i] = Integer.parseInt(sc.next());
			d += 1/(double)A[i];
		} int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
			A[i] = Integer.parseInt(sc.next());
			d += 1/(double)A[i];
		} {
			A[i] = Integer.parseInt(sc.next());
			d += 1/(double)A[i];
		} A[i] = Integer.parseInt(sc.next()) A[i] A i Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc d += 1/(double)A[i] d 1/(double)A[i] 1 (double)A[i] A i sc.close() sc.close sc System.out.println(1/d) System.out.println System.out System System.out 1/d 1 d String[] args args public class Main{

	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int N = Integer.parseInt(sc.nextLine());
		int[] A = new int[N];
		double d = 0;
		for(int i = 0;i < N;i++){
			A[i] = Integer.parseInt(sc.next());
			d += 1/(double)A[i];
		}
		sc.close();
		System.out.println(1/d);
	}
} public class Main{

	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int N = Integer.parseInt(sc.nextLine());
		int[] A = new int[N];
		double d = 0;
		for(int i = 0;i < N;i++){
			A[i] = Integer.parseInt(sc.next());
			d += 1/(double)A[i];
		}
		sc.close();
		System.out.println(1/d);
	}
} Main
import java.util.Scanner;

public class Main{

	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int N = Integer.parseInt(sc.nextLine());
		int[] A = new int[N];
		double d = 0;
		for(int i = 0;i < N;i++){
			A[i] = Integer.parseInt(sc.next());
			d += 1/(double)A[i];
		}
		sc.close();
		System.out.println(1/d);
	}
}