import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		try (Scanner scanner = new Scanner(System.in)) {
			int N = scanner.nextInt();
			double[] array = new double[N];

			double result = 0;
			for (int i = 0; i < N; i++) {
				array[i] = scanner.nextInt();
			}
			for (int i  = 0; i < N; i++) {
				double tmp = 1.0 / array[i];
				result += tmp;
			}

			System.out.println(1 / result);
		}
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {
		try (Scanner scanner = new Scanner(System.in)) {
			int N = scanner.nextInt();
			double[] array = new double[N];

			double result = 0;
			for (int i = 0; i < N; i++) {
				array[i] = scanner.nextInt();
			}
			for (int i  = 0; i < N; i++) {
				double tmp = 1.0 / array[i];
				result += tmp;
			}

			System.out.println(1 / result);
		}
	}
} Main public static void main(String[] args) {
		try (Scanner scanner = new Scanner(System.in)) {
			int N = scanner.nextInt();
			double[] array = new double[N];

			double result = 0;
			for (int i = 0; i < N; i++) {
				array[i] = scanner.nextInt();
			}
			for (int i  = 0; i < N; i++) {
				double tmp = 1.0 / array[i];
				result += tmp;
			}

			System.out.println(1 / result);
		}
	} main {
		try (Scanner scanner = new Scanner(System.in)) {
			int N = scanner.nextInt();
			double[] array = new double[N];

			double result = 0;
			for (int i = 0; i < N; i++) {
				array[i] = scanner.nextInt();
			}
			for (int i  = 0; i < N; i++) {
				double tmp = 1.0 / array[i];
				result += tmp;
			}

			System.out.println(1 / result);
		}
	} try (Scanner scanner = new Scanner(System.in)) {
			int N = scanner.nextInt();
			double[] array = new double[N];

			double result = 0;
			for (int i = 0; i < N; i++) {
				array[i] = scanner.nextInt();
			}
			for (int i  = 0; i < N; i++) {
				double tmp = 1.0 / array[i];
				result += tmp;
			}

			System.out.println(1 / result);
		} {
			int N = scanner.nextInt();
			double[] array = new double[N];

			double result = 0;
			for (int i = 0; i < N; i++) {
				array[i] = scanner.nextInt();
			}
			for (int i  = 0; i < N; i++) {
				double tmp = 1.0 / array[i];
				result += tmp;
			}

			System.out.println(1 / result);
		} Scanner scanner = new Scanner(System.in) Scanner scanner = new Scanner(System.in) new Scanner(System.in) int N = scanner.nextInt(); N scanner.nextInt() scanner.nextInt scanner double[] array = new double[N]; array new double[N] N double result = 0; result 0 for (int i = 0; i < N; i++) {
				array[i] = scanner.nextInt();
			} int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
				array[i] = scanner.nextInt();
			} {
				array[i] = scanner.nextInt();
			} array[i] = scanner.nextInt() array[i] array i scanner.nextInt() scanner.nextInt scanner for (int i  = 0; i < N; i++) {
				double tmp = 1.0 / array[i];
				result += tmp;
			} int i  = 0; int i  = 0; i 0 i < N i N i++ i++ i {
				double tmp = 1.0 / array[i];
				result += tmp;
			} {
				double tmp = 1.0 / array[i];
				result += tmp;
			} double tmp = 1.0 / array[i]; tmp 1.0 / array[i] 1.0 array[i] array i result += tmp result tmp System.out.println(1 / result) System.out.println System.out System System.out 1 / result 1 result String[] args args public class Main {
	public static void main(String[] args) {
		try (Scanner scanner = new Scanner(System.in)) {
			int N = scanner.nextInt();
			double[] array = new double[N];

			double result = 0;
			for (int i = 0; i < N; i++) {
				array[i] = scanner.nextInt();
			}
			for (int i  = 0; i < N; i++) {
				double tmp = 1.0 / array[i];
				result += tmp;
			}

			System.out.println(1 / result);
		}
	}
} public class Main {
	public static void main(String[] args) {
		try (Scanner scanner = new Scanner(System.in)) {
			int N = scanner.nextInt();
			double[] array = new double[N];

			double result = 0;
			for (int i = 0; i < N; i++) {
				array[i] = scanner.nextInt();
			}
			for (int i  = 0; i < N; i++) {
				double tmp = 1.0 / array[i];
				result += tmp;
			}

			System.out.println(1 / result);
		}
	}
} Main
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		try (Scanner scanner = new Scanner(System.in)) {
			int N = scanner.nextInt();
			double[] array = new double[N];

			double result = 0;
			for (int i = 0; i < N; i++) {
				array[i] = scanner.nextInt();
			}
			for (int i  = 0; i < N; i++) {
				double tmp = 1.0 / array[i];
				result += tmp;
			}

			System.out.println(1 / result);
		}
	}
}