import java.util.Scanner;

class Main{
  public static void main(String[] args){
    Scanner stdIn = new Scanner(System.in);
    int n = stdIn.nextInt();
    double[] numbers = new double[n];
    double sum = 0;
      for(int i = 0; i<n; i++){
        numbers[i] = 1.0/stdIn.nextInt();
        sum += numbers[i];
      }
    System.out.println(1/sum);
    stdIn.close();
  }
}
 import java.util.Scanner; Scanner java.util class Main{
  public static void main(String[] args){
    Scanner stdIn = new Scanner(System.in);
    int n = stdIn.nextInt();
    double[] numbers = new double[n];
    double sum = 0;
      for(int i = 0; i<n; i++){
        numbers[i] = 1.0/stdIn.nextInt();
        sum += numbers[i];
      }
    System.out.println(1/sum);
    stdIn.close();
  }
} Main public static void main(String[] args){
    Scanner stdIn = new Scanner(System.in);
    int n = stdIn.nextInt();
    double[] numbers = new double[n];
    double sum = 0;
      for(int i = 0; i<n; i++){
        numbers[i] = 1.0/stdIn.nextInt();
        sum += numbers[i];
      }
    System.out.println(1/sum);
    stdIn.close();
  } main {
    Scanner stdIn = new Scanner(System.in);
    int n = stdIn.nextInt();
    double[] numbers = new double[n];
    double sum = 0;
      for(int i = 0; i<n; i++){
        numbers[i] = 1.0/stdIn.nextInt();
        sum += numbers[i];
      }
    System.out.println(1/sum);
    stdIn.close();
  } Scanner stdIn = new Scanner(System.in); stdIn new Scanner(System.in) int n = stdIn.nextInt(); n stdIn.nextInt() stdIn.nextInt stdIn double[] numbers = new double[n]; numbers new double[n] n double sum = 0; sum 0 for(int i = 0; i<n; i++){
        numbers[i] = 1.0/stdIn.nextInt();
        sum += numbers[i];
      } int i = 0; int i = 0; i 0 i<n i n i++ i++ i {
        numbers[i] = 1.0/stdIn.nextInt();
        sum += numbers[i];
      } {
        numbers[i] = 1.0/stdIn.nextInt();
        sum += numbers[i];
      } numbers[i] = 1.0/stdIn.nextInt() numbers[i] numbers i 1.0/stdIn.nextInt() 1.0 stdIn.nextInt() stdIn.nextInt stdIn sum += numbers[i] sum numbers[i] numbers i System.out.println(1/sum) System.out.println System.out System System.out 1/sum 1 sum stdIn.close() stdIn.close stdIn String[] args args
import java.util.Scanner;

class Main{
  public static void main(String[] args){
    Scanner stdIn = new Scanner(System.in);
    int n = stdIn.nextInt();
    double[] numbers = new double[n];
    double sum = 0;
      for(int i = 0; i<n; i++){
        numbers[i] = 1.0/stdIn.nextInt();
        sum += numbers[i];
      }
    System.out.println(1/sum);
    stdIn.close();
  }
}

