import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        double sum = 0;
        double tmp;
        for(int i=0;i<n;i++){
            tmp = 1.0/Integer.parseInt(sc.next());
            sum+=tmp;
        }
        System.out.println(1.0/sum);
        sc.close();
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        double sum = 0;
        double tmp;
        for(int i=0;i<n;i++){
            tmp = 1.0/Integer.parseInt(sc.next());
            sum+=tmp;
        }
        System.out.println(1.0/sum);
        sc.close();
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        double sum = 0;
        double tmp;
        for(int i=0;i<n;i++){
            tmp = 1.0/Integer.parseInt(sc.next());
            sum+=tmp;
        }
        System.out.println(1.0/sum);
        sc.close();
    } main {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        double sum = 0;
        double tmp;
        for(int i=0;i<n;i++){
            tmp = 1.0/Integer.parseInt(sc.next());
            sum+=tmp;
        }
        System.out.println(1.0/sum);
        sc.close();
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = Integer.parseInt(sc.next()); n Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc double sum = 0; sum 0 double tmp; tmp for(int i=0;i<n;i++){
            tmp = 1.0/Integer.parseInt(sc.next());
            sum+=tmp;
        } int i=0; int i=0; i 0 i<n i n i++ i++ i {
            tmp = 1.0/Integer.parseInt(sc.next());
            sum+=tmp;
        } {
            tmp = 1.0/Integer.parseInt(sc.next());
            sum+=tmp;
        } tmp = 1.0/Integer.parseInt(sc.next()) tmp 1.0/Integer.parseInt(sc.next()) 1.0 Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc sum+=tmp sum tmp System.out.println(1.0/sum) System.out.println System.out System System.out 1.0/sum 1.0 sum sc.close() sc.close sc String[] args args public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        double sum = 0;
        double tmp;
        for(int i=0;i<n;i++){
            tmp = 1.0/Integer.parseInt(sc.next());
            sum+=tmp;
        }
        System.out.println(1.0/sum);
        sc.close();
    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        double sum = 0;
        double tmp;
        for(int i=0;i<n;i++){
            tmp = 1.0/Integer.parseInt(sc.next());
            sum+=tmp;
        }
        System.out.println(1.0/sum);
        sc.close();
    }
} Main
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        double sum = 0;
        double tmp;
        for(int i=0;i<n;i++){
            tmp = 1.0/Integer.parseInt(sc.next());
            sum+=tmp;
        }
        System.out.println(1.0/sum);
        sc.close();
    }
}