import java.util.Scanner;

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

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