import java.util.Scanner;

public class Main{
  public static void main(String[] args){
    Scanner sc=new Scanner(System.in);
    final int N=sc.nextInt();
    final int[] A=new int[N];
    for(int i=0;i<N;i++){
      A[i]=sc.nextInt();
    }
    double a=0.0;
    for(int i=0;i<N;i++){
      a+=(double)1/(double)A[i];
    }
    double answer=(double)1/(double)a;
    System.out.println(answer);
  }
} import java.util.Scanner; Scanner java.util public class Main{
  public static void main(String[] args){
    Scanner sc=new Scanner(System.in);
    final int N=sc.nextInt();
    final int[] A=new int[N];
    for(int i=0;i<N;i++){
      A[i]=sc.nextInt();
    }
    double a=0.0;
    for(int i=0;i<N;i++){
      a+=(double)1/(double)A[i];
    }
    double answer=(double)1/(double)a;
    System.out.println(answer);
  }
} Main public static void main(String[] args){
    Scanner sc=new Scanner(System.in);
    final int N=sc.nextInt();
    final int[] A=new int[N];
    for(int i=0;i<N;i++){
      A[i]=sc.nextInt();
    }
    double a=0.0;
    for(int i=0;i<N;i++){
      a+=(double)1/(double)A[i];
    }
    double answer=(double)1/(double)a;
    System.out.println(answer);
  } main {
    Scanner sc=new Scanner(System.in);
    final int N=sc.nextInt();
    final int[] A=new int[N];
    for(int i=0;i<N;i++){
      A[i]=sc.nextInt();
    }
    double a=0.0;
    for(int i=0;i<N;i++){
      a+=(double)1/(double)A[i];
    }
    double answer=(double)1/(double)a;
    System.out.println(answer);
  } Scanner sc=new Scanner(System.in); sc new Scanner(System.in) final int N=sc.nextInt(); N sc.nextInt() sc.nextInt sc final 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 a=0.0; a 0.0 for(int i=0;i<N;i++){
      a+=(double)1/(double)A[i];
    } int i=0; int i=0; i 0 i<N i N i++ i++ i {
      a+=(double)1/(double)A[i];
    } {
      a+=(double)1/(double)A[i];
    } a+=(double)1/(double)A[i] a (double)1/(double)A[i] (double)1 (double)A[i] A i double answer=(double)1/(double)a; answer (double)1/(double)a (double)1 (double)a System.out.println(answer) System.out.println System.out System System.out answer String[] args args public class Main{
  public static void main(String[] args){
    Scanner sc=new Scanner(System.in);
    final int N=sc.nextInt();
    final int[] A=new int[N];
    for(int i=0;i<N;i++){
      A[i]=sc.nextInt();
    }
    double a=0.0;
    for(int i=0;i<N;i++){
      a+=(double)1/(double)A[i];
    }
    double answer=(double)1/(double)a;
    System.out.println(answer);
  }
} public class Main{
  public static void main(String[] args){
    Scanner sc=new Scanner(System.in);
    final int N=sc.nextInt();
    final int[] A=new int[N];
    for(int i=0;i<N;i++){
      A[i]=sc.nextInt();
    }
    double a=0.0;
    for(int i=0;i<N;i++){
      a+=(double)1/(double)A[i];
    }
    double answer=(double)1/(double)a;
    System.out.println(answer);
  }
} Main
import java.util.Scanner;

public class Main{
  public static void main(String[] args){
    Scanner sc=new Scanner(System.in);
    final int N=sc.nextInt();
    final int[] A=new int[N];
    for(int i=0;i<N;i++){
      A[i]=sc.nextInt();
    }
    double a=0.0;
    for(int i=0;i<N;i++){
      a+=(double)1/(double)A[i];
    }
    double answer=(double)1/(double)a;
    System.out.println(answer);
  }
}
