import java.util.Scanner;

/**
 * https://atcoder.jp/contests/abc138/tasks/abc138_b
 */
public class Main {

  private static double answer(int[] hs) {
    double d = 0.0;
    for (int h : hs) {
      d += 1.0 / h;
    }

    return 1.0 / d;
  }

  public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    int n = scan.nextInt();
    int[] as = new int[n];
    for (int i = 0; i < n; i++) {
      as[i] = scan.nextInt();
    }
    scan.close();

    System.out.println(answer(as));
  }
} import java.util.Scanner; Scanner java.util public class Main {

  private static double answer(int[] hs) {
    double d = 0.0;
    for (int h : hs) {
      d += 1.0 / h;
    }

    return 1.0 / d;
  }

  public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    int n = scan.nextInt();
    int[] as = new int[n];
    for (int i = 0; i < n; i++) {
      as[i] = scan.nextInt();
    }
    scan.close();

    System.out.println(answer(as));
  }
} Main private static double answer(int[] hs) {
    double d = 0.0;
    for (int h : hs) {
      d += 1.0 / h;
    }

    return 1.0 / d;
  } answer {
    double d = 0.0;
    for (int h : hs) {
      d += 1.0 / h;
    }

    return 1.0 / d;
  } double d = 0.0; d 0.0 for (int h : hs) {
      d += 1.0 / h;
    } int h hs {
      d += 1.0 / h;
    } {
      d += 1.0 / h;
    } d += 1.0 / h d 1.0 / h 1.0 h return 1.0 / d; 1.0 / d 1.0 d int[] hs hs public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    int n = scan.nextInt();
    int[] as = new int[n];
    for (int i = 0; i < n; i++) {
      as[i] = scan.nextInt();
    }
    scan.close();

    System.out.println(answer(as));
  } main {
    Scanner scan = new Scanner(System.in);
    int n = scan.nextInt();
    int[] as = new int[n];
    for (int i = 0; i < n; i++) {
      as[i] = scan.nextInt();
    }
    scan.close();

    System.out.println(answer(as));
  } Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int n = scan.nextInt(); n scan.nextInt() scan.nextInt scan int[] as = new int[n]; as new int[n] n for (int i = 0; i < n; i++) {
      as[i] = scan.nextInt();
    } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
      as[i] = scan.nextInt();
    } {
      as[i] = scan.nextInt();
    } as[i] = scan.nextInt() as[i] as i scan.nextInt() scan.nextInt scan scan.close() scan.close scan System.out.println(answer(as)) System.out.println System.out System System.out answer(as) answer as String[] args args public class Main {

  private static double answer(int[] hs) {
    double d = 0.0;
    for (int h : hs) {
      d += 1.0 / h;
    }

    return 1.0 / d;
  }

  public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    int n = scan.nextInt();
    int[] as = new int[n];
    for (int i = 0; i < n; i++) {
      as[i] = scan.nextInt();
    }
    scan.close();

    System.out.println(answer(as));
  }
} public class Main {

  private static double answer(int[] hs) {
    double d = 0.0;
    for (int h : hs) {
      d += 1.0 / h;
    }

    return 1.0 / d;
  }

  public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    int n = scan.nextInt();
    int[] as = new int[n];
    for (int i = 0; i < n; i++) {
      as[i] = scan.nextInt();
    }
    scan.close();

    System.out.println(answer(as));
  }
} Main
import java.util.Scanner;

/**
 * https://atcoder.jp/contests/abc138/tasks/abc138_b
 */
public class Main {

  private static double answer(int[] hs) {
    double d = 0.0;
    for (int h : hs) {
      d += 1.0 / h;
    }

    return 1.0 / d;
  }

  public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    int n = scan.nextInt();
    int[] as = new int[n];
    for (int i = 0; i < n; i++) {
      as[i] = scan.nextInt();
    }
    scan.close();

    System.out.println(answer(as));
  }
}
