import java.util.Scanner;

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

    int N = scanner.nextInt();
    double m = 0.d;
    for (int i = 0; i < N; i++) {
      int a = scanner.nextInt();
      m += 1.d / a;
    }

    System.out.println(1.d / m);
  }
} import java.util.Scanner; Scanner java.util public class Main {
  public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);

    int N = scanner.nextInt();
    double m = 0.d;
    for (int i = 0; i < N; i++) {
      int a = scanner.nextInt();
      m += 1.d / a;
    }

    System.out.println(1.d / m);
  }
} Main public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);

    int N = scanner.nextInt();
    double m = 0.d;
    for (int i = 0; i < N; i++) {
      int a = scanner.nextInt();
      m += 1.d / a;
    }

    System.out.println(1.d / m);
  } main {
    Scanner scanner = new Scanner(System.in);

    int N = scanner.nextInt();
    double m = 0.d;
    for (int i = 0; i < N; i++) {
      int a = scanner.nextInt();
      m += 1.d / a;
    }

    System.out.println(1.d / m);
  } Scanner scanner = new Scanner(System.in); scanner new Scanner(System.in) int N = scanner.nextInt(); N scanner.nextInt() scanner.nextInt scanner double m = 0.d; m 0.d for (int i = 0; i < N; i++) {
      int a = scanner.nextInt();
      m += 1.d / a;
    } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
      int a = scanner.nextInt();
      m += 1.d / a;
    } {
      int a = scanner.nextInt();
      m += 1.d / a;
    } int a = scanner.nextInt(); a scanner.nextInt() scanner.nextInt scanner m += 1.d / a m 1.d / a 1.d a System.out.println(1.d / m) System.out.println System.out System System.out 1.d / m 1.d m String[] args args public class Main {
  public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);

    int N = scanner.nextInt();
    double m = 0.d;
    for (int i = 0; i < N; i++) {
      int a = scanner.nextInt();
      m += 1.d / a;
    }

    System.out.println(1.d / m);
  }
} public class Main {
  public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);

    int N = scanner.nextInt();
    double m = 0.d;
    for (int i = 0; i < N; i++) {
      int a = scanner.nextInt();
      m += 1.d / a;
    }

    System.out.println(1.d / m);
  }
} Main
import java.util.Scanner;

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

    int N = scanner.nextInt();
    double m = 0.d;
    for (int i = 0; i < N; i++) {
      int a = scanner.nextInt();
      m += 1.d / a;
    }

    System.out.println(1.d / m);
  }
}
