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

class Main {
	public static void main(String[] args) {
		try (Scanner sc = new Scanner(System.in)) {
			int size = sc.nextInt();
			BigDecimal divisor = BigDecimal.ZERO;
			for (int i = 0; i < size; i++) {
				BigDecimal bd = BigDecimal.ONE.setScale(15).divide(new BigDecimal(sc.nextInt()),
						BigDecimal.ROUND_HALF_UP);
				divisor = divisor.add(bd);
			}
			BigDecimal ans = BigDecimal.ONE.setScale(10).divide(divisor, BigDecimal.ROUND_HALF_UP);
			System.out.println(ans);
		}
	}
} import java.math.BigDecimal; BigDecimal java.math import java.util.Scanner; Scanner java.util class Main {
	public static void main(String[] args) {
		try (Scanner sc = new Scanner(System.in)) {
			int size = sc.nextInt();
			BigDecimal divisor = BigDecimal.ZERO;
			for (int i = 0; i < size; i++) {
				BigDecimal bd = BigDecimal.ONE.setScale(15).divide(new BigDecimal(sc.nextInt()),
						BigDecimal.ROUND_HALF_UP);
				divisor = divisor.add(bd);
			}
			BigDecimal ans = BigDecimal.ONE.setScale(10).divide(divisor, BigDecimal.ROUND_HALF_UP);
			System.out.println(ans);
		}
	}
} Main public static void main(String[] args) {
		try (Scanner sc = new Scanner(System.in)) {
			int size = sc.nextInt();
			BigDecimal divisor = BigDecimal.ZERO;
			for (int i = 0; i < size; i++) {
				BigDecimal bd = BigDecimal.ONE.setScale(15).divide(new BigDecimal(sc.nextInt()),
						BigDecimal.ROUND_HALF_UP);
				divisor = divisor.add(bd);
			}
			BigDecimal ans = BigDecimal.ONE.setScale(10).divide(divisor, BigDecimal.ROUND_HALF_UP);
			System.out.println(ans);
		}
	} main {
		try (Scanner sc = new Scanner(System.in)) {
			int size = sc.nextInt();
			BigDecimal divisor = BigDecimal.ZERO;
			for (int i = 0; i < size; i++) {
				BigDecimal bd = BigDecimal.ONE.setScale(15).divide(new BigDecimal(sc.nextInt()),
						BigDecimal.ROUND_HALF_UP);
				divisor = divisor.add(bd);
			}
			BigDecimal ans = BigDecimal.ONE.setScale(10).divide(divisor, BigDecimal.ROUND_HALF_UP);
			System.out.println(ans);
		}
	} try (Scanner sc = new Scanner(System.in)) {
			int size = sc.nextInt();
			BigDecimal divisor = BigDecimal.ZERO;
			for (int i = 0; i < size; i++) {
				BigDecimal bd = BigDecimal.ONE.setScale(15).divide(new BigDecimal(sc.nextInt()),
						BigDecimal.ROUND_HALF_UP);
				divisor = divisor.add(bd);
			}
			BigDecimal ans = BigDecimal.ONE.setScale(10).divide(divisor, BigDecimal.ROUND_HALF_UP);
			System.out.println(ans);
		} {
			int size = sc.nextInt();
			BigDecimal divisor = BigDecimal.ZERO;
			for (int i = 0; i < size; i++) {
				BigDecimal bd = BigDecimal.ONE.setScale(15).divide(new BigDecimal(sc.nextInt()),
						BigDecimal.ROUND_HALF_UP);
				divisor = divisor.add(bd);
			}
			BigDecimal ans = BigDecimal.ONE.setScale(10).divide(divisor, BigDecimal.ROUND_HALF_UP);
			System.out.println(ans);
		} Scanner sc = new Scanner(System.in) Scanner sc = new Scanner(System.in) new Scanner(System.in) int size = sc.nextInt(); size sc.nextInt() sc.nextInt sc BigDecimal divisor = BigDecimal.ZERO; divisor BigDecimal.ZERO BigDecimal BigDecimal.ZERO for (int i = 0; i < size; i++) {
				BigDecimal bd = BigDecimal.ONE.setScale(15).divide(new BigDecimal(sc.nextInt()),
						BigDecimal.ROUND_HALF_UP);
				divisor = divisor.add(bd);
			} int i = 0; int i = 0; i 0 i < size i size i++ i++ i {
				BigDecimal bd = BigDecimal.ONE.setScale(15).divide(new BigDecimal(sc.nextInt()),
						BigDecimal.ROUND_HALF_UP);
				divisor = divisor.add(bd);
			} {
				BigDecimal bd = BigDecimal.ONE.setScale(15).divide(new BigDecimal(sc.nextInt()),
						BigDecimal.ROUND_HALF_UP);
				divisor = divisor.add(bd);
			} BigDecimal bd = BigDecimal.ONE.setScale(15).divide(new BigDecimal(sc.nextInt()),
						BigDecimal.ROUND_HALF_UP); bd BigDecimal.ONE.setScale(15).divide(new BigDecimal(sc.nextInt()),
						BigDecimal.ROUND_HALF_UP) BigDecimal.ONE.setScale(15).divide BigDecimal.ONE.setScale(15) BigDecimal.ONE.setScale BigDecimal.ONE BigDecimal BigDecimal.ONE 15 new BigDecimal(sc.nextInt()) BigDecimal.ROUND_HALF_UP BigDecimal BigDecimal.ROUND_HALF_UP divisor = divisor.add(bd) divisor divisor.add(bd) divisor.add divisor bd BigDecimal ans = BigDecimal.ONE.setScale(10).divide(divisor, BigDecimal.ROUND_HALF_UP); ans BigDecimal.ONE.setScale(10).divide(divisor, BigDecimal.ROUND_HALF_UP) BigDecimal.ONE.setScale(10).divide BigDecimal.ONE.setScale(10) BigDecimal.ONE.setScale BigDecimal.ONE BigDecimal BigDecimal.ONE 10 divisor BigDecimal.ROUND_HALF_UP BigDecimal BigDecimal.ROUND_HALF_UP System.out.println(ans) System.out.println System.out System System.out ans String[] args args
import java.math.BigDecimal;
import java.util.Scanner;

class Main {
	public static void main(String[] args) {
		try (Scanner sc = new Scanner(System.in)) {
			int size = sc.nextInt();
			BigDecimal divisor = BigDecimal.ZERO;
			for (int i = 0; i < size; i++) {
				BigDecimal bd = BigDecimal.ONE.setScale(15).divide(new BigDecimal(sc.nextInt()),
						BigDecimal.ROUND_HALF_UP);
				divisor = divisor.add(bd);
			}
			BigDecimal ans = BigDecimal.ONE.setScale(10).divide(divisor, BigDecimal.ROUND_HALF_UP);
			System.out.println(ans);
		}
	}
}