import java.util.*;
import static java.lang.System.*;
import java.math.*;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double[] list = new double[n];
		for(int i = 0; i < n; i++)
			list[i] = sc.nextDouble();
		
		double sum = 0;
		for(int i = 0; i < n; i++) {
			double d = 1.0 / list[i];
			sum += d;
		}
		double ans = 1.0 / sum;
		out.println(ans);
	}
} import java.util.*; util.* java import static java.lang.System.*; import static java.lang.System.*; import static java.lang.System.*; import java.math.*; math.* java public class Main {

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

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

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double[] list = new double[n];
		for(int i = 0; i < n; i++)
			list[i] = sc.nextDouble();
		
		double sum = 0;
		for(int i = 0; i < n; i++) {
			double d = 1.0 / list[i];
			sum += d;
		}
		double ans = 1.0 / sum;
		out.println(ans);
	}
} Main
import java.util.*;
import static java.lang.System.*;
import java.math.*;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double[] list = new double[n];
		for(int i = 0; i < n; i++)
			list[i] = sc.nextDouble();
		
		double sum = 0;
		for(int i = 0; i < n; i++) {
			double d = 1.0 / list[i];
			sum += d;
		}
		double ans = 1.0 / sum;
		out.println(ans);
	}
}