import java.util.*;

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

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