import java.math.BigDecimal;
import java.util.Scanner;

public class Main {
	public static void main(String args[]){

		Scanner sc = new Scanner(System.in);

		BigDecimal result = new BigDecimal("1");
		int n = sc.nextInt();

		double sum = 0;

		for(int i=0;i < n;i++) {
			BigDecimal next = new BigDecimal(sc.nextInt());
			BigDecimal div = BigDecimal.ONE.divide(next,10,BigDecimal.ROUND_HALF_UP);
			sum += div.doubleValue();
		}

		System.out.println(BigDecimal.ONE.divide(new BigDecimal(sum),20,BigDecimal.ROUND_HALF_UP));

	}
} import java.math.BigDecimal; BigDecimal java.math import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String args[]){

		Scanner sc = new Scanner(System.in);

		BigDecimal result = new BigDecimal("1");
		int n = sc.nextInt();

		double sum = 0;

		for(int i=0;i < n;i++) {
			BigDecimal next = new BigDecimal(sc.nextInt());
			BigDecimal div = BigDecimal.ONE.divide(next,10,BigDecimal.ROUND_HALF_UP);
			sum += div.doubleValue();
		}

		System.out.println(BigDecimal.ONE.divide(new BigDecimal(sum),20,BigDecimal.ROUND_HALF_UP));

	}
} Main public static void main(String args[]){

		Scanner sc = new Scanner(System.in);

		BigDecimal result = new BigDecimal("1");
		int n = sc.nextInt();

		double sum = 0;

		for(int i=0;i < n;i++) {
			BigDecimal next = new BigDecimal(sc.nextInt());
			BigDecimal div = BigDecimal.ONE.divide(next,10,BigDecimal.ROUND_HALF_UP);
			sum += div.doubleValue();
		}

		System.out.println(BigDecimal.ONE.divide(new BigDecimal(sum),20,BigDecimal.ROUND_HALF_UP));

	} main {

		Scanner sc = new Scanner(System.in);

		BigDecimal result = new BigDecimal("1");
		int n = sc.nextInt();

		double sum = 0;

		for(int i=0;i < n;i++) {
			BigDecimal next = new BigDecimal(sc.nextInt());
			BigDecimal div = BigDecimal.ONE.divide(next,10,BigDecimal.ROUND_HALF_UP);
			sum += div.doubleValue();
		}

		System.out.println(BigDecimal.ONE.divide(new BigDecimal(sum),20,BigDecimal.ROUND_HALF_UP));

	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) BigDecimal result = new BigDecimal("1"); result new BigDecimal("1") int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc double sum = 0; sum 0 for(int i=0;i < n;i++) {
			BigDecimal next = new BigDecimal(sc.nextInt());
			BigDecimal div = BigDecimal.ONE.divide(next,10,BigDecimal.ROUND_HALF_UP);
			sum += div.doubleValue();
		} int i=0; int i=0; i 0 i < n i n i++ i++ i {
			BigDecimal next = new BigDecimal(sc.nextInt());
			BigDecimal div = BigDecimal.ONE.divide(next,10,BigDecimal.ROUND_HALF_UP);
			sum += div.doubleValue();
		} {
			BigDecimal next = new BigDecimal(sc.nextInt());
			BigDecimal div = BigDecimal.ONE.divide(next,10,BigDecimal.ROUND_HALF_UP);
			sum += div.doubleValue();
		} BigDecimal next = new BigDecimal(sc.nextInt()); next new BigDecimal(sc.nextInt()) BigDecimal div = BigDecimal.ONE.divide(next,10,BigDecimal.ROUND_HALF_UP); div BigDecimal.ONE.divide(next,10,BigDecimal.ROUND_HALF_UP) BigDecimal.ONE.divide BigDecimal.ONE BigDecimal BigDecimal.ONE next 10 BigDecimal.ROUND_HALF_UP BigDecimal BigDecimal.ROUND_HALF_UP sum += div.doubleValue() sum div.doubleValue() div.doubleValue div System.out.println(BigDecimal.ONE.divide(new BigDecimal(sum),20,BigDecimal.ROUND_HALF_UP)) System.out.println System.out System System.out BigDecimal.ONE.divide(new BigDecimal(sum),20,BigDecimal.ROUND_HALF_UP) BigDecimal.ONE.divide BigDecimal.ONE BigDecimal BigDecimal.ONE new BigDecimal(sum) 20 BigDecimal.ROUND_HALF_UP BigDecimal BigDecimal.ROUND_HALF_UP String args[] args public class Main {
	public static void main(String args[]){

		Scanner sc = new Scanner(System.in);

		BigDecimal result = new BigDecimal("1");
		int n = sc.nextInt();

		double sum = 0;

		for(int i=0;i < n;i++) {
			BigDecimal next = new BigDecimal(sc.nextInt());
			BigDecimal div = BigDecimal.ONE.divide(next,10,BigDecimal.ROUND_HALF_UP);
			sum += div.doubleValue();
		}

		System.out.println(BigDecimal.ONE.divide(new BigDecimal(sum),20,BigDecimal.ROUND_HALF_UP));

	}
} public class Main {
	public static void main(String args[]){

		Scanner sc = new Scanner(System.in);

		BigDecimal result = new BigDecimal("1");
		int n = sc.nextInt();

		double sum = 0;

		for(int i=0;i < n;i++) {
			BigDecimal next = new BigDecimal(sc.nextInt());
			BigDecimal div = BigDecimal.ONE.divide(next,10,BigDecimal.ROUND_HALF_UP);
			sum += div.doubleValue();
		}

		System.out.println(BigDecimal.ONE.divide(new BigDecimal(sum),20,BigDecimal.ROUND_HALF_UP));

	}
} Main
import java.math.BigDecimal;
import java.util.Scanner;

public class Main {
	public static void main(String args[]){

		Scanner sc = new Scanner(System.in);

		BigDecimal result = new BigDecimal("1");
		int n = sc.nextInt();

		double sum = 0;

		for(int i=0;i < n;i++) {
			BigDecimal next = new BigDecimal(sc.nextInt());
			BigDecimal div = BigDecimal.ONE.divide(next,10,BigDecimal.ROUND_HALF_UP);
			sum += div.doubleValue();
		}

		System.out.println(BigDecimal.ONE.divide(new BigDecimal(sum),20,BigDecimal.ROUND_HALF_UP));

	}
}