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

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		BigDecimal sum = BigDecimal.ZERO;
		BigDecimal multiSum = BigDecimal.ONE;
		BigDecimal let;
		MathContext mc = new MathContext(20,RoundingMode.HALF_UP);


		for (int i = 0; i < N; i++) {
			let = new BigDecimal(sc.next());
			sum = sum.add(BigDecimal.ONE.divide(let,mc));
		}

		System.out.println(BigDecimal.ONE.divide(sum,mc));

	}
}

 import java.math.BigDecimal; BigDecimal java.math import java.math.MathContext; MathContext java.math import java.math.RoundingMode; RoundingMode java.math import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		BigDecimal sum = BigDecimal.ZERO;
		BigDecimal multiSum = BigDecimal.ONE;
		BigDecimal let;
		MathContext mc = new MathContext(20,RoundingMode.HALF_UP);


		for (int i = 0; i < N; i++) {
			let = new BigDecimal(sc.next());
			sum = sum.add(BigDecimal.ONE.divide(let,mc));
		}

		System.out.println(BigDecimal.ONE.divide(sum,mc));

	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		BigDecimal sum = BigDecimal.ZERO;
		BigDecimal multiSum = BigDecimal.ONE;
		BigDecimal let;
		MathContext mc = new MathContext(20,RoundingMode.HALF_UP);


		for (int i = 0; i < N; i++) {
			let = new BigDecimal(sc.next());
			sum = sum.add(BigDecimal.ONE.divide(let,mc));
		}

		System.out.println(BigDecimal.ONE.divide(sum,mc));

	} main {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		BigDecimal sum = BigDecimal.ZERO;
		BigDecimal multiSum = BigDecimal.ONE;
		BigDecimal let;
		MathContext mc = new MathContext(20,RoundingMode.HALF_UP);


		for (int i = 0; i < N; i++) {
			let = new BigDecimal(sc.next());
			sum = sum.add(BigDecimal.ONE.divide(let,mc));
		}

		System.out.println(BigDecimal.ONE.divide(sum,mc));

	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc BigDecimal sum = BigDecimal.ZERO; sum BigDecimal.ZERO BigDecimal BigDecimal.ZERO BigDecimal multiSum = BigDecimal.ONE; multiSum BigDecimal.ONE BigDecimal BigDecimal.ONE BigDecimal let; let MathContext mc = new MathContext(20,RoundingMode.HALF_UP); mc new MathContext(20,RoundingMode.HALF_UP) for (int i = 0; i < N; i++) {
			let = new BigDecimal(sc.next());
			sum = sum.add(BigDecimal.ONE.divide(let,mc));
		} int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
			let = new BigDecimal(sc.next());
			sum = sum.add(BigDecimal.ONE.divide(let,mc));
		} {
			let = new BigDecimal(sc.next());
			sum = sum.add(BigDecimal.ONE.divide(let,mc));
		} let = new BigDecimal(sc.next()) let new BigDecimal(sc.next()) sum = sum.add(BigDecimal.ONE.divide(let,mc)) sum sum.add(BigDecimal.ONE.divide(let,mc)) sum.add sum BigDecimal.ONE.divide(let,mc) BigDecimal.ONE.divide BigDecimal.ONE BigDecimal BigDecimal.ONE let mc System.out.println(BigDecimal.ONE.divide(sum,mc)) System.out.println System.out System System.out BigDecimal.ONE.divide(sum,mc) BigDecimal.ONE.divide BigDecimal.ONE BigDecimal BigDecimal.ONE sum mc String[] args args public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		BigDecimal sum = BigDecimal.ZERO;
		BigDecimal multiSum = BigDecimal.ONE;
		BigDecimal let;
		MathContext mc = new MathContext(20,RoundingMode.HALF_UP);


		for (int i = 0; i < N; i++) {
			let = new BigDecimal(sc.next());
			sum = sum.add(BigDecimal.ONE.divide(let,mc));
		}

		System.out.println(BigDecimal.ONE.divide(sum,mc));

	}
} public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		BigDecimal sum = BigDecimal.ZERO;
		BigDecimal multiSum = BigDecimal.ONE;
		BigDecimal let;
		MathContext mc = new MathContext(20,RoundingMode.HALF_UP);


		for (int i = 0; i < N; i++) {
			let = new BigDecimal(sc.next());
			sum = sum.add(BigDecimal.ONE.divide(let,mc));
		}

		System.out.println(BigDecimal.ONE.divide(sum,mc));

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

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		BigDecimal sum = BigDecimal.ZERO;
		BigDecimal multiSum = BigDecimal.ONE;
		BigDecimal let;
		MathContext mc = new MathContext(20,RoundingMode.HALF_UP);


		for (int i = 0; i < N; i++) {
			let = new BigDecimal(sc.next());
			sum = sum.add(BigDecimal.ONE.divide(let,mc));
		}

		System.out.println(BigDecimal.ONE.divide(sum,mc));

	}
}


