import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int[] a = new int[n];
    for (int i = 0; i < n; i++) {
      a[i] = sc.nextInt();
    }

    double gyaku = 0;
      for (int i = 0; i < n; i++) {
      gyaku += 1d / (double)a[i];
    }

    System.out.println(1d / gyaku);
  }
} import java.util.*; util.* java public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int[] a = new int[n];
    for (int i = 0; i < n; i++) {
      a[i] = sc.nextInt();
    }

    double gyaku = 0;
      for (int i = 0; i < n; i++) {
      gyaku += 1d / (double)a[i];
    }

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

    double gyaku = 0;
      for (int i = 0; i < n; i++) {
      gyaku += 1d / (double)a[i];
    }

    System.out.println(1d / gyaku);
  } main {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int[] a = new int[n];
    for (int i = 0; i < n; i++) {
      a[i] = sc.nextInt();
    }

    double gyaku = 0;
      for (int i = 0; i < n; i++) {
      gyaku += 1d / (double)a[i];
    }

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

    double gyaku = 0;
      for (int i = 0; i < n; i++) {
      gyaku += 1d / (double)a[i];
    }

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

    double gyaku = 0;
      for (int i = 0; i < n; i++) {
      gyaku += 1d / (double)a[i];
    }

    System.out.println(1d / gyaku);
  }
} Main
import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int[] a = new int[n];
    for (int i = 0; i < n; i++) {
      a[i] = sc.nextInt();
    }

    double gyaku = 0;
      for (int i = 0; i < n; i++) {
      gyaku += 1d / (double)a[i];
    }

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