import java.util.*;

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

    double sum = 0;
    for (int i = 0; i < N; i++) {
      sum += 1 / scn.nextDouble();
    }

    System.out.println(1 / sum);
  }

} import java.util.*; util.* java public class Main {
  public static void main(String[] args) {
    Scanner scn = new Scanner(System.in);
    int N = scn.nextInt();

    double sum = 0;
    for (int i = 0; i < N; i++) {
      sum += 1 / scn.nextDouble();
    }

    System.out.println(1 / sum);
  }

} Main public static void main(String[] args) {
    Scanner scn = new Scanner(System.in);
    int N = scn.nextInt();

    double sum = 0;
    for (int i = 0; i < N; i++) {
      sum += 1 / scn.nextDouble();
    }

    System.out.println(1 / sum);
  } main {
    Scanner scn = new Scanner(System.in);
    int N = scn.nextInt();

    double sum = 0;
    for (int i = 0; i < N; i++) {
      sum += 1 / scn.nextDouble();
    }

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

    double sum = 0;
    for (int i = 0; i < N; i++) {
      sum += 1 / scn.nextDouble();
    }

    System.out.println(1 / sum);
  }

} public class Main {
  public static void main(String[] args) {
    Scanner scn = new Scanner(System.in);
    int N = scn.nextInt();

    double sum = 0;
    for (int i = 0; i < N; i++) {
      sum += 1 / scn.nextDouble();
    }

    System.out.println(1 / sum);
  }

} Main
import java.util.*;

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

    double sum = 0;
    for (int i = 0; i < N; i++) {
      sum += 1 / scn.nextDouble();
    }

    System.out.println(1 / sum);
  }

}
