import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
         Scanner sc = new Scanner(System.in);
         int n  = sc.nextInt();
         int a[] = new int[n];
         for(int i= 0; i<n ; i++){
                 a[i] = sc.nextInt();
         }
         double j = 0.0;
         for(int i= 0 ; i<n ; i++){
             j += (double)1/a[i];
         }
         System.out.println(1/j);
         
    }
} 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();
         int a[] = new int[n];
         for(int i= 0; i<n ; i++){
                 a[i] = sc.nextInt();
         }
         double j = 0.0;
         for(int i= 0 ; i<n ; i++){
             j += (double)1/a[i];
         }
         System.out.println(1/j);
         
    }
} Main public static void main(String[] args) {
         Scanner sc = new Scanner(System.in);
         int n  = sc.nextInt();
         int a[] = new int[n];
         for(int i= 0; i<n ; i++){
                 a[i] = sc.nextInt();
         }
         double j = 0.0;
         for(int i= 0 ; i<n ; i++){
             j += (double)1/a[i];
         }
         System.out.println(1/j);
         
    } main {
         Scanner sc = new Scanner(System.in);
         int n  = sc.nextInt();
         int a[] = new int[n];
         for(int i= 0; i<n ; i++){
                 a[i] = sc.nextInt();
         }
         double j = 0.0;
         for(int i= 0 ; i<n ; i++){
             j += (double)1/a[i];
         }
         System.out.println(1/j);
         
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n  = sc.nextInt(); n sc.nextInt() sc.nextInt sc int a[] = new int[n]; a new int[n] n for(int i= 0; i<n ; i++){
                 a[i] = sc.nextInt();
         } int i= 0; int i= 0; i 0 i<n i n i++ i++ i {
                 a[i] = sc.nextInt();
         } {
                 a[i] = sc.nextInt();
         } a[i] = sc.nextInt() a[i] a i sc.nextInt() sc.nextInt sc double j = 0.0; j 0.0 for(int i= 0 ; i<n ; i++){
             j += (double)1/a[i];
         } int i= 0 ; int i= 0 ; i 0 i<n i n i++ i++ i {
             j += (double)1/a[i];
         } {
             j += (double)1/a[i];
         } j += (double)1/a[i] j (double)1/a[i] (double)1 a[i] a i System.out.println(1/j) System.out.println System.out System System.out 1/j 1 j String[] args args public class Main {
    public static void main(String[] args) {
         Scanner sc = new Scanner(System.in);
         int n  = sc.nextInt();
         int a[] = new int[n];
         for(int i= 0; i<n ; i++){
                 a[i] = sc.nextInt();
         }
         double j = 0.0;
         for(int i= 0 ; i<n ; i++){
             j += (double)1/a[i];
         }
         System.out.println(1/j);
         
    }
} public class Main {
    public static void main(String[] args) {
         Scanner sc = new Scanner(System.in);
         int n  = sc.nextInt();
         int a[] = new int[n];
         for(int i= 0; i<n ; i++){
                 a[i] = sc.nextInt();
         }
         double j = 0.0;
         for(int i= 0 ; i<n ; i++){
             j += (double)1/a[i];
         }
         System.out.println(1/j);
         
    }
} Main
import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
         Scanner sc = new Scanner(System.in);
         int n  = sc.nextInt();
         int a[] = new int[n];
         for(int i= 0; i<n ; i++){
                 a[i] = sc.nextInt();
         }
         double j = 0.0;
         for(int i= 0 ; i<n ; i++){
             j += (double)1/a[i];
         }
         System.out.println(1/j);
         
    }
}
