import java.util.*;

public class Main {
	public static void main(String[] args) {
		MyScanner in = new MyScanner();
		int n = in.nextInt();
		double[] a = in.nextDoubleArray(n);
		double sum = 0;
		
		for (int i = 0; i < a.length; i++) {
			sum += 1/a[i];
		}
		
		System.out.println(1/sum);
		
	}
	
	private static class MyScanner {
		static Scanner sc = new Scanner(System.in);
		
		String next() {
			return sc.next();
		}
		
		String nextLine() {
			return sc.nextLine();
		}
		
		int nextInt() {
			return Integer.parseInt(sc.next());
		}
		
		double nextDouble() {
			return Double.parseDouble(next());
		}
		
		long nextLong() {
			return Long.parseLong(next());
		}
		
		int[] nextIntArray(int n) {
			int[] array = new int[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextInt();
			}
			return array;
		}
		
		double[] nextDoubleArray(int n) {
			double[] array = new double[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextDouble();
			}
			return array;
		}
		
		long[] nextLongArray(int n) {
			long[] array = new long[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextLong();
			}
			return array;
		}
		
	}
} import java.util.*; util.* java public class Main {
	public static void main(String[] args) {
		MyScanner in = new MyScanner();
		int n = in.nextInt();
		double[] a = in.nextDoubleArray(n);
		double sum = 0;
		
		for (int i = 0; i < a.length; i++) {
			sum += 1/a[i];
		}
		
		System.out.println(1/sum);
		
	}
	
	private static class MyScanner {
		static Scanner sc = new Scanner(System.in);
		
		String next() {
			return sc.next();
		}
		
		String nextLine() {
			return sc.nextLine();
		}
		
		int nextInt() {
			return Integer.parseInt(sc.next());
		}
		
		double nextDouble() {
			return Double.parseDouble(next());
		}
		
		long nextLong() {
			return Long.parseLong(next());
		}
		
		int[] nextIntArray(int n) {
			int[] array = new int[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextInt();
			}
			return array;
		}
		
		double[] nextDoubleArray(int n) {
			double[] array = new double[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextDouble();
			}
			return array;
		}
		
		long[] nextLongArray(int n) {
			long[] array = new long[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextLong();
			}
			return array;
		}
		
	}
} Main public static void main(String[] args) {
		MyScanner in = new MyScanner();
		int n = in.nextInt();
		double[] a = in.nextDoubleArray(n);
		double sum = 0;
		
		for (int i = 0; i < a.length; i++) {
			sum += 1/a[i];
		}
		
		System.out.println(1/sum);
		
	} main {
		MyScanner in = new MyScanner();
		int n = in.nextInt();
		double[] a = in.nextDoubleArray(n);
		double sum = 0;
		
		for (int i = 0; i < a.length; i++) {
			sum += 1/a[i];
		}
		
		System.out.println(1/sum);
		
	} MyScanner in = new MyScanner(); in new MyScanner() int n = in.nextInt(); n in.nextInt() in.nextInt in double[] a = in.nextDoubleArray(n); a in.nextDoubleArray(n) in.nextDoubleArray in n double sum = 0; sum 0 for (int i = 0; i < a.length; i++) {
			sum += 1/a[i];
		} int i = 0; int i = 0; i 0 i < a.length i a.length a a.length i++ i++ i {
			sum += 1/a[i];
		} {
			sum += 1/a[i];
		} sum += 1/a[i] sum 1/a[i] 1 a[i] a i System.out.println(1/sum) System.out.println System.out System System.out 1/sum 1 sum String[] args args private static class MyScanner {
		static Scanner sc = new Scanner(System.in);
		
		String next() {
			return sc.next();
		}
		
		String nextLine() {
			return sc.nextLine();
		}
		
		int nextInt() {
			return Integer.parseInt(sc.next());
		}
		
		double nextDouble() {
			return Double.parseDouble(next());
		}
		
		long nextLong() {
			return Long.parseLong(next());
		}
		
		int[] nextIntArray(int n) {
			int[] array = new int[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextInt();
			}
			return array;
		}
		
		double[] nextDoubleArray(int n) {
			double[] array = new double[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextDouble();
			}
			return array;
		}
		
		long[] nextLongArray(int n) {
			long[] array = new long[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextLong();
			}
			return array;
		}
		
	} MyScanner static Scanner sc = new Scanner(System.in); sc new Scanner(System.in) String next() {
			return sc.next();
		} next {
			return sc.next();
		} return sc.next(); sc.next() sc.next sc String nextLine() {
			return sc.nextLine();
		} nextLine {
			return sc.nextLine();
		} return sc.nextLine(); sc.nextLine() sc.nextLine sc int nextInt() {
			return Integer.parseInt(sc.next());
		} nextInt {
			return Integer.parseInt(sc.next());
		} return Integer.parseInt(sc.next()); Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc double nextDouble() {
			return Double.parseDouble(next());
		} nextDouble {
			return Double.parseDouble(next());
		} return Double.parseDouble(next()); Double.parseDouble(next()) Double.parseDouble Double next() next long nextLong() {
			return Long.parseLong(next());
		} nextLong {
			return Long.parseLong(next());
		} return Long.parseLong(next()); Long.parseLong(next()) Long.parseLong Long next() next int[] nextIntArray(int n) {
			int[] array = new int[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextInt();
			}
			return array;
		} nextIntArray {
			int[] array = new int[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextInt();
			}
			return array;
		} int[] array = new int[n]; array new int[n] n for (int i = 0; i < n; i++) {
				array[i] = nextInt();
			} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
				array[i] = nextInt();
			} {
				array[i] = nextInt();
			} array[i] = nextInt() array[i] array i nextInt() nextInt return array; array int n n double[] nextDoubleArray(int n) {
			double[] array = new double[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextDouble();
			}
			return array;
		} nextDoubleArray {
			double[] array = new double[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextDouble();
			}
			return array;
		} double[] array = new double[n]; array new double[n] n for (int i = 0; i < n; i++) {
				array[i] = nextDouble();
			} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
				array[i] = nextDouble();
			} {
				array[i] = nextDouble();
			} array[i] = nextDouble() array[i] array i nextDouble() nextDouble return array; array int n n long[] nextLongArray(int n) {
			long[] array = new long[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextLong();
			}
			return array;
		} nextLongArray {
			long[] array = new long[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextLong();
			}
			return array;
		} long[] array = new long[n]; array new long[n] n for (int i = 0; i < n; i++) {
				array[i] = nextLong();
			} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
				array[i] = nextLong();
			} {
				array[i] = nextLong();
			} array[i] = nextLong() array[i] array i nextLong() nextLong return array; array int n n public class Main {
	public static void main(String[] args) {
		MyScanner in = new MyScanner();
		int n = in.nextInt();
		double[] a = in.nextDoubleArray(n);
		double sum = 0;
		
		for (int i = 0; i < a.length; i++) {
			sum += 1/a[i];
		}
		
		System.out.println(1/sum);
		
	}
	
	private static class MyScanner {
		static Scanner sc = new Scanner(System.in);
		
		String next() {
			return sc.next();
		}
		
		String nextLine() {
			return sc.nextLine();
		}
		
		int nextInt() {
			return Integer.parseInt(sc.next());
		}
		
		double nextDouble() {
			return Double.parseDouble(next());
		}
		
		long nextLong() {
			return Long.parseLong(next());
		}
		
		int[] nextIntArray(int n) {
			int[] array = new int[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextInt();
			}
			return array;
		}
		
		double[] nextDoubleArray(int n) {
			double[] array = new double[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextDouble();
			}
			return array;
		}
		
		long[] nextLongArray(int n) {
			long[] array = new long[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextLong();
			}
			return array;
		}
		
	}
} public class Main {
	public static void main(String[] args) {
		MyScanner in = new MyScanner();
		int n = in.nextInt();
		double[] a = in.nextDoubleArray(n);
		double sum = 0;
		
		for (int i = 0; i < a.length; i++) {
			sum += 1/a[i];
		}
		
		System.out.println(1/sum);
		
	}
	
	private static class MyScanner {
		static Scanner sc = new Scanner(System.in);
		
		String next() {
			return sc.next();
		}
		
		String nextLine() {
			return sc.nextLine();
		}
		
		int nextInt() {
			return Integer.parseInt(sc.next());
		}
		
		double nextDouble() {
			return Double.parseDouble(next());
		}
		
		long nextLong() {
			return Long.parseLong(next());
		}
		
		int[] nextIntArray(int n) {
			int[] array = new int[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextInt();
			}
			return array;
		}
		
		double[] nextDoubleArray(int n) {
			double[] array = new double[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextDouble();
			}
			return array;
		}
		
		long[] nextLongArray(int n) {
			long[] array = new long[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextLong();
			}
			return array;
		}
		
	}
} Main
import java.util.*;

public class Main {
	public static void main(String[] args) {
		MyScanner in = new MyScanner();
		int n = in.nextInt();
		double[] a = in.nextDoubleArray(n);
		double sum = 0;
		
		for (int i = 0; i < a.length; i++) {
			sum += 1/a[i];
		}
		
		System.out.println(1/sum);
		
	}
	
	private static class MyScanner {
		static Scanner sc = new Scanner(System.in);
		
		String next() {
			return sc.next();
		}
		
		String nextLine() {
			return sc.nextLine();
		}
		
		int nextInt() {
			return Integer.parseInt(sc.next());
		}
		
		double nextDouble() {
			return Double.parseDouble(next());
		}
		
		long nextLong() {
			return Long.parseLong(next());
		}
		
		int[] nextIntArray(int n) {
			int[] array = new int[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextInt();
			}
			return array;
		}
		
		double[] nextDoubleArray(int n) {
			double[] array = new double[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextDouble();
			}
			return array;
		}
		
		long[] nextLongArray(int n) {
			long[] array = new long[n];
			for (int i = 0; i < n; i++) {
				array[i] = nextLong();
			}
			return array;
		}
		
	}
}
