import java.util.*;

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

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