import java.util.*;
public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] num = new int[n];
        double sum = 0;


        for(int i = 0;i<n;i++){
            num[i] = sc.nextInt();
            sum += (double)1/num[i];
        }
        //System.out.println(sum);

        System.out.println((double)1/sum);



    }
} import java.util.*; util.* java public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] num = new int[n];
        double sum = 0;


        for(int i = 0;i<n;i++){
            num[i] = sc.nextInt();
            sum += (double)1/num[i];
        }
        //System.out.println(sum);

        System.out.println((double)1/sum);



    }
} Main public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] num = new int[n];
        double sum = 0;


        for(int i = 0;i<n;i++){
            num[i] = sc.nextInt();
            sum += (double)1/num[i];
        }
        //System.out.println(sum);

        System.out.println((double)1/sum);



    } main {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] num = new int[n];
        double sum = 0;


        for(int i = 0;i<n;i++){
            num[i] = sc.nextInt();
            sum += (double)1/num[i];
        }
        //System.out.println(sum);

        System.out.println((double)1/sum);



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


        for(int i = 0;i<n;i++){
            num[i] = sc.nextInt();
            sum += (double)1/num[i];
        }
        //System.out.println(sum);

        System.out.println((double)1/sum);



    }
} public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] num = new int[n];
        double sum = 0;


        for(int i = 0;i<n;i++){
            num[i] = sc.nextInt();
            sum += (double)1/num[i];
        }
        //System.out.println(sum);

        System.out.println((double)1/sum);



    }
} Main
import java.util.*;
public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] num = new int[n];
        double sum = 0;


        for(int i = 0;i<n;i++){
            num[i] = sc.nextInt();
            sum += (double)1/num[i];
        }
        //System.out.println(sum);

        System.out.println((double)1/sum);



    }
}
