import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;

public class Main {
  public static void main(String[] args) throws IOException {
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    int n = Integer.parseInt(in.readLine());
    String[] a_str = in.readLine().split(" ");
    double[] a = new double[n];
    double sum = 0;
    
    for(int i = 0; i < n; i++){
      a[i] = (double)1/Integer.parseInt(a_str[i]);
      sum += a[i];
    }
    System.out.println((double)1/sum);
  }
} import java.io.BufferedReader; BufferedReader java.io import java.io.InputStreamReader; InputStreamReader java.io import java.io.IOException; IOException java.io public class Main {
  public static void main(String[] args) throws IOException {
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    int n = Integer.parseInt(in.readLine());
    String[] a_str = in.readLine().split(" ");
    double[] a = new double[n];
    double sum = 0;
    
    for(int i = 0; i < n; i++){
      a[i] = (double)1/Integer.parseInt(a_str[i]);
      sum += a[i];
    }
    System.out.println((double)1/sum);
  }
} Main public static void main(String[] args) throws IOException {
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    int n = Integer.parseInt(in.readLine());
    String[] a_str = in.readLine().split(" ");
    double[] a = new double[n];
    double sum = 0;
    
    for(int i = 0; i < n; i++){
      a[i] = (double)1/Integer.parseInt(a_str[i]);
      sum += a[i];
    }
    System.out.println((double)1/sum);
  } main {
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    int n = Integer.parseInt(in.readLine());
    String[] a_str = in.readLine().split(" ");
    double[] a = new double[n];
    double sum = 0;
    
    for(int i = 0; i < n; i++){
      a[i] = (double)1/Integer.parseInt(a_str[i]);
      sum += a[i];
    }
    System.out.println((double)1/sum);
  } BufferedReader in = new BufferedReader(new InputStreamReader(System.in)); in new BufferedReader(new InputStreamReader(System.in)) int n = Integer.parseInt(in.readLine()); n Integer.parseInt(in.readLine()) Integer.parseInt Integer in.readLine() in.readLine in String[] a_str = in.readLine().split(" "); a_str in.readLine().split(" ") in.readLine().split in.readLine() in.readLine in " " double[] a = new double[n]; a new double[n] n double sum = 0; sum 0 for(int i = 0; i < n; i++){
      a[i] = (double)1/Integer.parseInt(a_str[i]);
      sum += a[i];
    } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
      a[i] = (double)1/Integer.parseInt(a_str[i]);
      sum += a[i];
    } {
      a[i] = (double)1/Integer.parseInt(a_str[i]);
      sum += a[i];
    } a[i] = (double)1/Integer.parseInt(a_str[i]) a[i] a i (double)1/Integer.parseInt(a_str[i]) (double)1 Integer.parseInt(a_str[i]) Integer.parseInt Integer a_str[i] a_str i sum += a[i] sum a[i] a 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) throws IOException {
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    int n = Integer.parseInt(in.readLine());
    String[] a_str = in.readLine().split(" ");
    double[] a = new double[n];
    double sum = 0;
    
    for(int i = 0; i < n; i++){
      a[i] = (double)1/Integer.parseInt(a_str[i]);
      sum += a[i];
    }
    System.out.println((double)1/sum);
  }
} public class Main {
  public static void main(String[] args) throws IOException {
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    int n = Integer.parseInt(in.readLine());
    String[] a_str = in.readLine().split(" ");
    double[] a = new double[n];
    double sum = 0;
    
    for(int i = 0; i < n; i++){
      a[i] = (double)1/Integer.parseInt(a_str[i]);
      sum += a[i];
    }
    System.out.println((double)1/sum);
  }
} Main
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;

public class Main {
  public static void main(String[] args) throws IOException {
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    int n = Integer.parseInt(in.readLine());
    String[] a_str = in.readLine().split(" ");
    double[] a = new double[n];
    double sum = 0;
    
    for(int i = 0; i < n; i++){
      a[i] = (double)1/Integer.parseInt(a_str[i]);
      sum += a[i];
    }
    System.out.println((double)1/sum);
  }
}