// package app;

import java.math.BigDecimal;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.Objects;
import java.util.Scanner;
import java.util.stream.Stream;

public class Main {

    public static void main(final String[] args) throws Exception {

        Scanner sc = new Scanner(System.in);

        // long N = sc.nextLong();
        // long K = sc.nextLong();
        int N = sc.nextInt();

        double[] A = new double[N];

        for (int i = 0; i < N; i++) {
            A[i] = sc.nextDouble();
        }

        double result = 0;

        for (int i = 0; i < N; i++) {
            result += 1.0 / A[i];
        }

        result = 1.0 / result;

        System.out.println(result);

    }
} import java.math.BigDecimal; BigDecimal java.math import java.util.Arrays; Arrays java.util import java.util.HashMap; HashMap java.util import java.util.Map; Map java.util import java.util.Objects; Objects java.util import java.util.Scanner; Scanner java.util import java.util.stream.Stream; Stream java.util.stream public class Main {

    public static void main(final String[] args) throws Exception {

        Scanner sc = new Scanner(System.in);

        // long N = sc.nextLong();
        // long K = sc.nextLong();
        int N = sc.nextInt();

        double[] A = new double[N];

        for (int i = 0; i < N; i++) {
            A[i] = sc.nextDouble();
        }

        double result = 0;

        for (int i = 0; i < N; i++) {
            result += 1.0 / A[i];
        }

        result = 1.0 / result;

        System.out.println(result);

    }
} Main public static void main(final String[] args) throws Exception {

        Scanner sc = new Scanner(System.in);

        // long N = sc.nextLong();
        // long K = sc.nextLong();
        int N = sc.nextInt();

        double[] A = new double[N];

        for (int i = 0; i < N; i++) {
            A[i] = sc.nextDouble();
        }

        double result = 0;

        for (int i = 0; i < N; i++) {
            result += 1.0 / A[i];
        }

        result = 1.0 / result;

        System.out.println(result);

    } main {

        Scanner sc = new Scanner(System.in);

        // long N = sc.nextLong();
        // long K = sc.nextLong();
        int N = sc.nextInt();

        double[] A = new double[N];

        for (int i = 0; i < N; i++) {
            A[i] = sc.nextDouble();
        }

        double result = 0;

        for (int i = 0; i < N; i++) {
            result += 1.0 / A[i];
        }

        result = 1.0 / result;

        System.out.println(result);

    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc double[] A = new double[N]; A new double[N] N for (int i = 0; i < N; i++) {
            A[i] = sc.nextDouble();
        } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
            A[i] = sc.nextDouble();
        } {
            A[i] = sc.nextDouble();
        } A[i] = sc.nextDouble() A[i] A i sc.nextDouble() sc.nextDouble sc double result = 0; result 0 for (int i = 0; i < N; i++) {
            result += 1.0 / A[i];
        } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
            result += 1.0 / A[i];
        } {
            result += 1.0 / A[i];
        } result += 1.0 / A[i] result 1.0 / A[i] 1.0 A[i] A i result = 1.0 / result result 1.0 / result 1.0 result System.out.println(result) System.out.println System.out System System.out result final String[] args args public class Main {

    public static void main(final String[] args) throws Exception {

        Scanner sc = new Scanner(System.in);

        // long N = sc.nextLong();
        // long K = sc.nextLong();
        int N = sc.nextInt();

        double[] A = new double[N];

        for (int i = 0; i < N; i++) {
            A[i] = sc.nextDouble();
        }

        double result = 0;

        for (int i = 0; i < N; i++) {
            result += 1.0 / A[i];
        }

        result = 1.0 / result;

        System.out.println(result);

    }
} public class Main {

    public static void main(final String[] args) throws Exception {

        Scanner sc = new Scanner(System.in);

        // long N = sc.nextLong();
        // long K = sc.nextLong();
        int N = sc.nextInt();

        double[] A = new double[N];

        for (int i = 0; i < N; i++) {
            A[i] = sc.nextDouble();
        }

        double result = 0;

        for (int i = 0; i < N; i++) {
            result += 1.0 / A[i];
        }

        result = 1.0 / result;

        System.out.println(result);

    }
} Main
// package app;

import java.math.BigDecimal;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.Objects;
import java.util.Scanner;
import java.util.stream.Stream;

public class Main {

    public static void main(final String[] args) throws Exception {

        Scanner sc = new Scanner(System.in);

        // long N = sc.nextLong();
        // long K = sc.nextLong();
        int N = sc.nextInt();

        double[] A = new double[N];

        for (int i = 0; i < N; i++) {
            A[i] = sc.nextDouble();
        }

        double result = 0;

        for (int i = 0; i < N; i++) {
            result += 1.0 / A[i];
        }

        result = 1.0 / result;

        System.out.println(result);

    }
}