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

public class Main {
	public static void main(String[] args) {
		try (Scanner sc = new Scanner(System.in)) {
			int n = Integer.parseInt(sc.nextLine());
			BigDecimal mother = new BigDecimal(0);
			for (int i = 0; i < n; i++) {
				mother = mother.add(new BigDecimal(1).divide(sc.nextBigDecimal(), 8, BigDecimal.ROUND_HALF_DOWN));
			}
			System.out.println(new BigDecimal(1).divide(mother, 8, BigDecimal.ROUND_HALF_DOWN));
		}
	}
} import java.math.BigDecimal; BigDecimal java.math import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {
		try (Scanner sc = new Scanner(System.in)) {
			int n = Integer.parseInt(sc.nextLine());
			BigDecimal mother = new BigDecimal(0);
			for (int i = 0; i < n; i++) {
				mother = mother.add(new BigDecimal(1).divide(sc.nextBigDecimal(), 8, BigDecimal.ROUND_HALF_DOWN));
			}
			System.out.println(new BigDecimal(1).divide(mother, 8, BigDecimal.ROUND_HALF_DOWN));
		}
	}
} Main public static void main(String[] args) {
		try (Scanner sc = new Scanner(System.in)) {
			int n = Integer.parseInt(sc.nextLine());
			BigDecimal mother = new BigDecimal(0);
			for (int i = 0; i < n; i++) {
				mother = mother.add(new BigDecimal(1).divide(sc.nextBigDecimal(), 8, BigDecimal.ROUND_HALF_DOWN));
			}
			System.out.println(new BigDecimal(1).divide(mother, 8, BigDecimal.ROUND_HALF_DOWN));
		}
	} main {
		try (Scanner sc = new Scanner(System.in)) {
			int n = Integer.parseInt(sc.nextLine());
			BigDecimal mother = new BigDecimal(0);
			for (int i = 0; i < n; i++) {
				mother = mother.add(new BigDecimal(1).divide(sc.nextBigDecimal(), 8, BigDecimal.ROUND_HALF_DOWN));
			}
			System.out.println(new BigDecimal(1).divide(mother, 8, BigDecimal.ROUND_HALF_DOWN));
		}
	} try (Scanner sc = new Scanner(System.in)) {
			int n = Integer.parseInt(sc.nextLine());
			BigDecimal mother = new BigDecimal(0);
			for (int i = 0; i < n; i++) {
				mother = mother.add(new BigDecimal(1).divide(sc.nextBigDecimal(), 8, BigDecimal.ROUND_HALF_DOWN));
			}
			System.out.println(new BigDecimal(1).divide(mother, 8, BigDecimal.ROUND_HALF_DOWN));
		} {
			int n = Integer.parseInt(sc.nextLine());
			BigDecimal mother = new BigDecimal(0);
			for (int i = 0; i < n; i++) {
				mother = mother.add(new BigDecimal(1).divide(sc.nextBigDecimal(), 8, BigDecimal.ROUND_HALF_DOWN));
			}
			System.out.println(new BigDecimal(1).divide(mother, 8, BigDecimal.ROUND_HALF_DOWN));
		} Scanner sc = new Scanner(System.in) Scanner sc = new Scanner(System.in) new Scanner(System.in) int n = Integer.parseInt(sc.nextLine()); n Integer.parseInt(sc.nextLine()) Integer.parseInt Integer sc.nextLine() sc.nextLine sc BigDecimal mother = new BigDecimal(0); mother new BigDecimal(0) for (int i = 0; i < n; i++) {
				mother = mother.add(new BigDecimal(1).divide(sc.nextBigDecimal(), 8, BigDecimal.ROUND_HALF_DOWN));
			} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
				mother = mother.add(new BigDecimal(1).divide(sc.nextBigDecimal(), 8, BigDecimal.ROUND_HALF_DOWN));
			} {
				mother = mother.add(new BigDecimal(1).divide(sc.nextBigDecimal(), 8, BigDecimal.ROUND_HALF_DOWN));
			} mother = mother.add(new BigDecimal(1).divide(sc.nextBigDecimal(), 8, BigDecimal.ROUND_HALF_DOWN)) mother mother.add(new BigDecimal(1).divide(sc.nextBigDecimal(), 8, BigDecimal.ROUND_HALF_DOWN)) mother.add mother new BigDecimal(1).divide(sc.nextBigDecimal(), 8, BigDecimal.ROUND_HALF_DOWN) new BigDecimal(1).divide new BigDecimal(1) sc.nextBigDecimal() sc.nextBigDecimal sc 8 BigDecimal.ROUND_HALF_DOWN BigDecimal BigDecimal.ROUND_HALF_DOWN System.out.println(new BigDecimal(1).divide(mother, 8, BigDecimal.ROUND_HALF_DOWN)) System.out.println System.out System System.out new BigDecimal(1).divide(mother, 8, BigDecimal.ROUND_HALF_DOWN) new BigDecimal(1).divide new BigDecimal(1) mother 8 BigDecimal.ROUND_HALF_DOWN BigDecimal BigDecimal.ROUND_HALF_DOWN String[] args args public class Main {
	public static void main(String[] args) {
		try (Scanner sc = new Scanner(System.in)) {
			int n = Integer.parseInt(sc.nextLine());
			BigDecimal mother = new BigDecimal(0);
			for (int i = 0; i < n; i++) {
				mother = mother.add(new BigDecimal(1).divide(sc.nextBigDecimal(), 8, BigDecimal.ROUND_HALF_DOWN));
			}
			System.out.println(new BigDecimal(1).divide(mother, 8, BigDecimal.ROUND_HALF_DOWN));
		}
	}
} public class Main {
	public static void main(String[] args) {
		try (Scanner sc = new Scanner(System.in)) {
			int n = Integer.parseInt(sc.nextLine());
			BigDecimal mother = new BigDecimal(0);
			for (int i = 0; i < n; i++) {
				mother = mother.add(new BigDecimal(1).divide(sc.nextBigDecimal(), 8, BigDecimal.ROUND_HALF_DOWN));
			}
			System.out.println(new BigDecimal(1).divide(mother, 8, BigDecimal.ROUND_HALF_DOWN));
		}
	}
} Main
import java.math.BigDecimal;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		try (Scanner sc = new Scanner(System.in)) {
			int n = Integer.parseInt(sc.nextLine());
			BigDecimal mother = new BigDecimal(0);
			for (int i = 0; i < n; i++) {
				mother = mother.add(new BigDecimal(1).divide(sc.nextBigDecimal(), 8, BigDecimal.ROUND_HALF_DOWN));
			}
			System.out.println(new BigDecimal(1).divide(mother, 8, BigDecimal.ROUND_HALF_DOWN));
		}
	}
}