import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        double[] a = new double[n];
        double sum = 0;
        for(int i = 0; i < n; i++){
            a[i] = 1 / scan.nextDouble();
            sum += a[i];
        }
        System.out.println(1 / sum);
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        double[] a = new double[n];
        double sum = 0;
        for(int i = 0; i < n; i++){
            a[i] = 1 / scan.nextDouble();
            sum += a[i];
        }
        System.out.println(1 / sum);
    }
} Main public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        double[] a = new double[n];
        double sum = 0;
        for(int i = 0; i < n; i++){
            a[i] = 1 / scan.nextDouble();
            sum += a[i];
        }
        System.out.println(1 / sum);
    } main {
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        double[] a = new double[n];
        double sum = 0;
        for(int i = 0; i < n; i++){
            a[i] = 1 / scan.nextDouble();
            sum += a[i];
        }
        System.out.println(1 / sum);
    } Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int n = scan.nextInt(); n scan.nextInt() scan.nextInt scan double[] a = new double[n]; a new double[n] n double sum = 0; sum 0 for(int i = 0; i < n; i++){
            a[i] = 1 / scan.nextDouble();
            sum += a[i];
        } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
            a[i] = 1 / scan.nextDouble();
            sum += a[i];
        } {
            a[i] = 1 / scan.nextDouble();
            sum += a[i];
        } a[i] = 1 / scan.nextDouble() a[i] a i 1 / scan.nextDouble() 1 scan.nextDouble() scan.nextDouble scan sum += a[i] sum a[i] a i System.out.println(1 / sum) System.out.println System.out System System.out 1 / sum 1 sum String[] args args public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        double[] a = new double[n];
        double sum = 0;
        for(int i = 0; i < n; i++){
            a[i] = 1 / scan.nextDouble();
            sum += a[i];
        }
        System.out.println(1 / sum);
    }
} public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        double[] a = new double[n];
        double sum = 0;
        for(int i = 0; i < n; i++){
            a[i] = 1 / scan.nextDouble();
            sum += a[i];
        }
        System.out.println(1 / sum);
    }
} Main
import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        double[] a = new double[n];
        double sum = 0;
        for(int i = 0; i < n; i++){
            a[i] = 1 / scan.nextDouble();
            sum += a[i];
        }
        System.out.println(1 / sum);
    }
}