import java.util.*;

public class Main {
  public static void main(String[] args) {
    @SuppressWarnings("resource")
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    double tmp = 0.0;
    
    for (int i = 0; i< N;i++) {
     tmp += (1.0 / sc.nextDouble());
    }
    
    System.out.println(1.0 / tmp);
  }
} import java.util.*; util.* java public class Main {
  public static void main(String[] args) {
    @SuppressWarnings("resource")
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    double tmp = 0.0;
    
    for (int i = 0; i< N;i++) {
     tmp += (1.0 / sc.nextDouble());
    }
    
    System.out.println(1.0 / tmp);
  }
} Main public static void main(String[] args) {
    @SuppressWarnings("resource")
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    double tmp = 0.0;
    
    for (int i = 0; i< N;i++) {
     tmp += (1.0 / sc.nextDouble());
    }
    
    System.out.println(1.0 / tmp);
  } main {
    @SuppressWarnings("resource")
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    double tmp = 0.0;
    
    for (int i = 0; i< N;i++) {
     tmp += (1.0 / sc.nextDouble());
    }
    
    System.out.println(1.0 / tmp);
  } @SuppressWarnings("resource")
    Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc double tmp = 0.0; tmp 0.0 for (int i = 0; i< N;i++) {
     tmp += (1.0 / sc.nextDouble());
    } int i = 0; int i = 0; i 0 i< N i N i++ i++ i {
     tmp += (1.0 / sc.nextDouble());
    } {
     tmp += (1.0 / sc.nextDouble());
    } tmp += (1.0 / sc.nextDouble()) tmp (1.0 / sc.nextDouble()) 1.0 sc.nextDouble() sc.nextDouble sc System.out.println(1.0 / tmp) System.out.println System.out System System.out 1.0 / tmp 1.0 tmp String[] args args public class Main {
  public static void main(String[] args) {
    @SuppressWarnings("resource")
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    double tmp = 0.0;
    
    for (int i = 0; i< N;i++) {
     tmp += (1.0 / sc.nextDouble());
    }
    
    System.out.println(1.0 / tmp);
  }
} public class Main {
  public static void main(String[] args) {
    @SuppressWarnings("resource")
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    double tmp = 0.0;
    
    for (int i = 0; i< N;i++) {
     tmp += (1.0 / sc.nextDouble());
    }
    
    System.out.println(1.0 / tmp);
  }
} Main
import java.util.*;

public class Main {
  public static void main(String[] args) {
    @SuppressWarnings("resource")
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    double tmp = 0.0;
    
    for (int i = 0; i< N;i++) {
     tmp += (1.0 / sc.nextDouble());
    }
    
    System.out.println(1.0 / tmp);
  }
}