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