//Resisotrs in Parallel
import java.util.*;
public class Main{
  public static void main(String[] args) {
    Scanner sc=new Scanner(System.in);
    int n=sc.nextInt();
    double data[]=new double[1000];

    for (int i=0;i<n;i++) {
      data[i]=sc.nextDouble();
    }

    /*for (int i=0;i<n;i++) {
      System.out.println(data[i]);
    }*/

    double oki=0;

    for (int i=0;i<n;i++) {
      oki=oki+(1.0/data[i]);
    }

    System.out.println(1.0/oki);

  }
} import java.util.*; util.* java public class Main{
  public static void main(String[] args) {
    Scanner sc=new Scanner(System.in);
    int n=sc.nextInt();
    double data[]=new double[1000];

    for (int i=0;i<n;i++) {
      data[i]=sc.nextDouble();
    }

    /*for (int i=0;i<n;i++) {
      System.out.println(data[i]);
    }*/

    double oki=0;

    for (int i=0;i<n;i++) {
      oki=oki+(1.0/data[i]);
    }

    System.out.println(1.0/oki);

  }
} Main public static void main(String[] args) {
    Scanner sc=new Scanner(System.in);
    int n=sc.nextInt();
    double data[]=new double[1000];

    for (int i=0;i<n;i++) {
      data[i]=sc.nextDouble();
    }

    /*for (int i=0;i<n;i++) {
      System.out.println(data[i]);
    }*/

    double oki=0;

    for (int i=0;i<n;i++) {
      oki=oki+(1.0/data[i]);
    }

    System.out.println(1.0/oki);

  } main {
    Scanner sc=new Scanner(System.in);
    int n=sc.nextInt();
    double data[]=new double[1000];

    for (int i=0;i<n;i++) {
      data[i]=sc.nextDouble();
    }

    /*for (int i=0;i<n;i++) {
      System.out.println(data[i]);
    }*/

    double oki=0;

    for (int i=0;i<n;i++) {
      oki=oki+(1.0/data[i]);
    }

    System.out.println(1.0/oki);

  } Scanner sc=new Scanner(System.in); sc new Scanner(System.in) int n=sc.nextInt(); n sc.nextInt() sc.nextInt sc double data[]=new double[1000]; data new double[1000] 1000 for (int i=0;i<n;i++) {
      data[i]=sc.nextDouble();
    } int i=0; int i=0; i 0 i<n i n i++ i++ i {
      data[i]=sc.nextDouble();
    } {
      data[i]=sc.nextDouble();
    } data[i]=sc.nextDouble() data[i] data i sc.nextDouble() sc.nextDouble sc double oki=0; oki 0 for (int i=0;i<n;i++) {
      oki=oki+(1.0/data[i]);
    } int i=0; int i=0; i 0 i<n i n i++ i++ i {
      oki=oki+(1.0/data[i]);
    } {
      oki=oki+(1.0/data[i]);
    } oki=oki+(1.0/data[i]) oki oki+(1.0/data[i]) oki (1.0/data[i]) 1.0 data[i] data i System.out.println(1.0/oki) System.out.println System.out System System.out 1.0/oki 1.0 oki String[] args args public class Main{
  public static void main(String[] args) {
    Scanner sc=new Scanner(System.in);
    int n=sc.nextInt();
    double data[]=new double[1000];

    for (int i=0;i<n;i++) {
      data[i]=sc.nextDouble();
    }

    /*for (int i=0;i<n;i++) {
      System.out.println(data[i]);
    }*/

    double oki=0;

    for (int i=0;i<n;i++) {
      oki=oki+(1.0/data[i]);
    }

    System.out.println(1.0/oki);

  }
} public class Main{
  public static void main(String[] args) {
    Scanner sc=new Scanner(System.in);
    int n=sc.nextInt();
    double data[]=new double[1000];

    for (int i=0;i<n;i++) {
      data[i]=sc.nextDouble();
    }

    /*for (int i=0;i<n;i++) {
      System.out.println(data[i]);
    }*/

    double oki=0;

    for (int i=0;i<n;i++) {
      oki=oki+(1.0/data[i]);
    }

    System.out.println(1.0/oki);

  }
} Main
//Resisotrs in Parallel
import java.util.*;
public class Main{
  public static void main(String[] args) {
    Scanner sc=new Scanner(System.in);
    int n=sc.nextInt();
    double data[]=new double[1000];

    for (int i=0;i<n;i++) {
      data[i]=sc.nextDouble();
    }

    /*for (int i=0;i<n;i++) {
      System.out.println(data[i]);
    }*/

    double oki=0;

    for (int i=0;i<n;i++) {
      oki=oki+(1.0/data[i]);
    }

    System.out.println(1.0/oki);

  }
}
