import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
    
    Scanner sc = new Scanner(System.in);

      int N = sc.nextInt();
      double []A = new double [N];
      double sum =0.0;
      
      //整数格納→1/整数→それを足す
      for(int i =0; i<N; i++){
      	A[i] = sc.nextInt();
        sum += 1.0/A[i];
      }
      //"１/逆数の和"を出力
      System.out.println(1.0/sum);
    }
} 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();
      double []A = new double [N];
      double sum =0.0;
      
      //整数格納→1/整数→それを足す
      for(int i =0; i<N; i++){
      	A[i] = sc.nextInt();
        sum += 1.0/A[i];
      }
      //"１/逆数の和"を出力
      System.out.println(1.0/sum);
    }
} Main public static void main(String[] args) {
    
    Scanner sc = new Scanner(System.in);

      int N = sc.nextInt();
      double []A = new double [N];
      double sum =0.0;
      
      //整数格納→1/整数→それを足す
      for(int i =0; i<N; i++){
      	A[i] = sc.nextInt();
        sum += 1.0/A[i];
      }
      //"１/逆数の和"を出力
      System.out.println(1.0/sum);
    } main {
    
    Scanner sc = new Scanner(System.in);

      int N = sc.nextInt();
      double []A = new double [N];
      double sum =0.0;
      
      //整数格納→1/整数→それを足す
      for(int i =0; i<N; i++){
      	A[i] = sc.nextInt();
        sum += 1.0/A[i];
      }
      //"１/逆数の和"を出力
      System.out.println(1.0/sum);
    } 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 double sum =0.0; sum 0.0 for(int i =0; i<N; i++){
      	A[i] = sc.nextInt();
        sum += 1.0/A[i];
      } int i =0; int i =0; i 0 i<N i N i++ i++ i {
      	A[i] = sc.nextInt();
        sum += 1.0/A[i];
      } {
      	A[i] = sc.nextInt();
        sum += 1.0/A[i];
      } A[i] = sc.nextInt() A[i] A i sc.nextInt() sc.nextInt sc sum += 1.0/A[i] sum 1.0/A[i] 1.0 A[i] A i System.out.println(1.0/sum) System.out.println System.out System System.out 1.0/sum 1.0 sum String[] args args public class Main {
    public static void main(String[] args) {
    
    Scanner sc = new Scanner(System.in);

      int N = sc.nextInt();
      double []A = new double [N];
      double sum =0.0;
      
      //整数格納→1/整数→それを足す
      for(int i =0; i<N; i++){
      	A[i] = sc.nextInt();
        sum += 1.0/A[i];
      }
      //"１/逆数の和"を出力
      System.out.println(1.0/sum);
    }
} public class Main {
    public static void main(String[] args) {
    
    Scanner sc = new Scanner(System.in);

      int N = sc.nextInt();
      double []A = new double [N];
      double sum =0.0;
      
      //整数格納→1/整数→それを足す
      for(int i =0; i<N; i++){
      	A[i] = sc.nextInt();
        sum += 1.0/A[i];
      }
      //"１/逆数の和"を出力
      System.out.println(1.0/sum);
    }
} Main
import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
    
    Scanner sc = new Scanner(System.in);

      int N = sc.nextInt();
      double []A = new double [N];
      double sum =0.0;
      
      //整数格納→1/整数→それを足す
      for(int i =0; i<N; i++){
      	A[i] = sc.nextInt();
        sum += 1.0/A[i];
      }
      //"１/逆数の和"を出力
      System.out.println(1.0/sum);
    }
}