import java.util.Scanner;

public class Main {

  public static void main(String[] args) {

    Scanner sc = new Scanner(System.in);

    int n = sc.nextInt();
    double ans = 0;

    for (int i = 0; i < n; i++) {
      double a = sc.nextInt();
      ans += 1.0 / a;
    }

    System.out.println(1.0 / ans);

  }
} 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 ans = 0;

    for (int i = 0; i < n; i++) {
      double a = sc.nextInt();
      ans += 1.0 / a;
    }

    System.out.println(1.0 / ans);

  }
} Main public static void main(String[] args) {

    Scanner sc = new Scanner(System.in);

    int n = sc.nextInt();
    double ans = 0;

    for (int i = 0; i < n; i++) {
      double a = sc.nextInt();
      ans += 1.0 / a;
    }

    System.out.println(1.0 / ans);

  } main {

    Scanner sc = new Scanner(System.in);

    int n = sc.nextInt();
    double ans = 0;

    for (int i = 0; i < n; i++) {
      double a = sc.nextInt();
      ans += 1.0 / a;
    }

    System.out.println(1.0 / ans);

  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc double ans = 0; ans 0 for (int i = 0; i < n; i++) {
      double a = sc.nextInt();
      ans += 1.0 / a;
    } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
      double a = sc.nextInt();
      ans += 1.0 / a;
    } {
      double a = sc.nextInt();
      ans += 1.0 / a;
    } double a = sc.nextInt(); a sc.nextInt() sc.nextInt sc ans += 1.0 / a ans 1.0 / a 1.0 a System.out.println(1.0 / ans) System.out.println System.out System System.out 1.0 / ans 1.0 ans String[] args args public class Main {

  public static void main(String[] args) {

    Scanner sc = new Scanner(System.in);

    int n = sc.nextInt();
    double ans = 0;

    for (int i = 0; i < n; i++) {
      double a = sc.nextInt();
      ans += 1.0 / a;
    }

    System.out.println(1.0 / ans);

  }
} public class Main {

  public static void main(String[] args) {

    Scanner sc = new Scanner(System.in);

    int n = sc.nextInt();
    double ans = 0;

    for (int i = 0; i < n; i++) {
      double a = sc.nextInt();
      ans += 1.0 / a;
    }

    System.out.println(1.0 / ans);

  }
} 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 ans = 0;

    for (int i = 0; i < n; i++) {
      double a = sc.nextInt();
      ans += 1.0 / a;
    }

    System.out.println(1.0 / ans);

  }
}