import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		StringBuilder sb = new StringBuilder();
		int n = sc.nextInt();
		double ans = 0;
		int a;
		for (int i = 0; i < n; i++) {
			a = sc.nextInt();
			ans += 1.0 / a;
		}
		ans = 1.0 / ans;
		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);
		StringBuilder sb = new StringBuilder();
		int n = sc.nextInt();
		double ans = 0;
		int a;
		for (int i = 0; i < n; i++) {
			a = sc.nextInt();
			ans += 1.0 / a;
		}
		ans = 1.0 / ans;
		System.out.println(ans);
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		StringBuilder sb = new StringBuilder();
		int n = sc.nextInt();
		double ans = 0;
		int a;
		for (int i = 0; i < n; i++) {
			a = sc.nextInt();
			ans += 1.0 / a;
		}
		ans = 1.0 / ans;
		System.out.println(ans);
	} main {
		Scanner sc = new Scanner(System.in);
		StringBuilder sb = new StringBuilder();
		int n = sc.nextInt();
		double ans = 0;
		int a;
		for (int i = 0; i < n; i++) {
			a = sc.nextInt();
			ans += 1.0 / a;
		}
		ans = 1.0 / ans;
		System.out.println(ans);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) StringBuilder sb = new StringBuilder(); sb new StringBuilder() int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc double ans = 0; ans 0 int a; a for (int i = 0; i < n; i++) {
			a = sc.nextInt();
			ans += 1.0 / a;
		} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
			a = sc.nextInt();
			ans += 1.0 / a;
		} {
			a = sc.nextInt();
			ans += 1.0 / a;
		} a = sc.nextInt() a sc.nextInt() sc.nextInt sc ans += 1.0 / a ans 1.0 / a 1.0 a ans = 1.0 / ans ans 1.0 / ans 1.0 ans 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);
		StringBuilder sb = new StringBuilder();
		int n = sc.nextInt();
		double ans = 0;
		int a;
		for (int i = 0; i < n; i++) {
			a = sc.nextInt();
			ans += 1.0 / a;
		}
		ans = 1.0 / ans;
		System.out.println(ans);
	}
} public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		StringBuilder sb = new StringBuilder();
		int n = sc.nextInt();
		double ans = 0;
		int a;
		for (int i = 0; i < n; i++) {
			a = sc.nextInt();
			ans += 1.0 / a;
		}
		ans = 1.0 / ans;
		System.out.println(ans);
	}
} Main
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		StringBuilder sb = new StringBuilder();
		int n = sc.nextInt();
		double ans = 0;
		int a;
		for (int i = 0; i < n; i++) {
			a = sc.nextInt();
			ans += 1.0 / a;
		}
		ans = 1.0 / ans;
		System.out.println(ans);
	}
}
