import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();

		int[] numArray = new int[n];

		for(int i = 0; i < n; i++){
			numArray[i] = sc.nextInt();
		}

		double sum = 0;
		for(int i = 0; i < n; i++){
			sum += 1.0/numArray[i];
		}

		System.out.println(1.0/sum);
	}
} import java.util.*; util.* java public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();

		int[] numArray = new int[n];

		for(int i = 0; i < n; i++){
			numArray[i] = sc.nextInt();
		}

		double sum = 0;
		for(int i = 0; i < n; i++){
			sum += 1.0/numArray[i];
		}

		System.out.println(1.0/sum);
	}
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();

		int[] numArray = new int[n];

		for(int i = 0; i < n; i++){
			numArray[i] = sc.nextInt();
		}

		double sum = 0;
		for(int i = 0; i < n; i++){
			sum += 1.0/numArray[i];
		}

		System.out.println(1.0/sum);
	} main {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();

		int[] numArray = new int[n];

		for(int i = 0; i < n; i++){
			numArray[i] = sc.nextInt();
		}

		double sum = 0;
		for(int i = 0; i < n; i++){
			sum += 1.0/numArray[i];
		}

		System.out.println(1.0/sum);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int[] numArray = new int[n]; numArray new int[n] n for(int i = 0; i < n; i++){
			numArray[i] = sc.nextInt();
		} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
			numArray[i] = sc.nextInt();
		} {
			numArray[i] = sc.nextInt();
		} numArray[i] = sc.nextInt() numArray[i] numArray i sc.nextInt() sc.nextInt sc double sum = 0; sum 0 for(int i = 0; i < n; i++){
			sum += 1.0/numArray[i];
		} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
			sum += 1.0/numArray[i];
		} {
			sum += 1.0/numArray[i];
		} sum += 1.0/numArray[i] sum 1.0/numArray[i] 1.0 numArray[i] numArray i System.out.println(1.0/sum) System.out.println System.out System System.out 1.0/sum 1.0 sum String[] args args public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();

		int[] numArray = new int[n];

		for(int i = 0; i < n; i++){
			numArray[i] = sc.nextInt();
		}

		double sum = 0;
		for(int i = 0; i < n; i++){
			sum += 1.0/numArray[i];
		}

		System.out.println(1.0/sum);
	}
} public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();

		int[] numArray = new int[n];

		for(int i = 0; i < n; i++){
			numArray[i] = sc.nextInt();
		}

		double sum = 0;
		for(int i = 0; i < n; i++){
			sum += 1.0/numArray[i];
		}

		System.out.println(1.0/sum);
	}
} Main
import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();

		int[] numArray = new int[n];

		for(int i = 0; i < n; i++){
			numArray[i] = sc.nextInt();
		}

		double sum = 0;
		for(int i = 0; i < n; i++){
			sum += 1.0/numArray[i];
		}

		System.out.println(1.0/sum);
	}
}