import java.util.*;
public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    double[] arr = new double[N];
    double A = 0;
    double B = 1;
    for (int i=0;i<N;i++){
      double n = scan.nextDouble();
      arr[i] = n;
      A+=n;
      B*=n;
    }
    double a = 0;
    for (int i=0;i<N;i++){
      a += B/arr[i];
    }
    System.out.println(B/a);
  }
} import java.util.*; util.* java public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    double[] arr = new double[N];
    double A = 0;
    double B = 1;
    for (int i=0;i<N;i++){
      double n = scan.nextDouble();
      arr[i] = n;
      A+=n;
      B*=n;
    }
    double a = 0;
    for (int i=0;i<N;i++){
      a += B/arr[i];
    }
    System.out.println(B/a);
  }
} Main public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    double[] arr = new double[N];
    double A = 0;
    double B = 1;
    for (int i=0;i<N;i++){
      double n = scan.nextDouble();
      arr[i] = n;
      A+=n;
      B*=n;
    }
    double a = 0;
    for (int i=0;i<N;i++){
      a += B/arr[i];
    }
    System.out.println(B/a);
  } main {
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    double[] arr = new double[N];
    double A = 0;
    double B = 1;
    for (int i=0;i<N;i++){
      double n = scan.nextDouble();
      arr[i] = n;
      A+=n;
      B*=n;
    }
    double a = 0;
    for (int i=0;i<N;i++){
      a += B/arr[i];
    }
    System.out.println(B/a);
  } Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int N = scan.nextInt(); N scan.nextInt() scan.nextInt scan double[] arr = new double[N]; arr new double[N] N double A = 0; A 0 double B = 1; B 1 for (int i=0;i<N;i++){
      double n = scan.nextDouble();
      arr[i] = n;
      A+=n;
      B*=n;
    } int i=0; int i=0; i 0 i<N i N i++ i++ i {
      double n = scan.nextDouble();
      arr[i] = n;
      A+=n;
      B*=n;
    } {
      double n = scan.nextDouble();
      arr[i] = n;
      A+=n;
      B*=n;
    } double n = scan.nextDouble(); n scan.nextDouble() scan.nextDouble scan arr[i] = n arr[i] arr i n A+=n A n B*=n B n double a = 0; a 0 for (int i=0;i<N;i++){
      a += B/arr[i];
    } int i=0; int i=0; i 0 i<N i N i++ i++ i {
      a += B/arr[i];
    } {
      a += B/arr[i];
    } a += B/arr[i] a B/arr[i] B arr[i] arr i System.out.println(B/a) System.out.println System.out System System.out B/a B a String[] args args public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    double[] arr = new double[N];
    double A = 0;
    double B = 1;
    for (int i=0;i<N;i++){
      double n = scan.nextDouble();
      arr[i] = n;
      A+=n;
      B*=n;
    }
    double a = 0;
    for (int i=0;i<N;i++){
      a += B/arr[i];
    }
    System.out.println(B/a);
  }
} public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    double[] arr = new double[N];
    double A = 0;
    double B = 1;
    for (int i=0;i<N;i++){
      double n = scan.nextDouble();
      arr[i] = n;
      A+=n;
      B*=n;
    }
    double a = 0;
    for (int i=0;i<N;i++){
      a += B/arr[i];
    }
    System.out.println(B/a);
  }
} Main
import java.util.*;
public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    double[] arr = new double[N];
    double A = 0;
    double B = 1;
    for (int i=0;i<N;i++){
      double n = scan.nextDouble();
      arr[i] = n;
      A+=n;
      B*=n;
    }
    double a = 0;
    for (int i=0;i<N;i++){
      a += B/arr[i];
    }
    System.out.println(B/a);
  }
}