import java.util.Scanner;

public class Main {

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

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

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

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

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

} public class Main {

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

} Main
import java.util.Scanner;

public class Main {

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

}
/*
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*/
