import java.util.Scanner;
class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    double sum = 0;
    for(int i=0; i<N; i++){
      sum += 1.0/sc.nextInt();
    }
    sum = 1.0/sum;
 
    System.out.print(sum);
  }
} import java.util.Scanner; Scanner java.util class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    double sum = 0;
    for(int i=0; i<N; i++){
      sum += 1.0/sc.nextInt();
    }
    sum = 1.0/sum;
 
    System.out.print(sum);
  }
} Main public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    double sum = 0;
    for(int i=0; i<N; i++){
      sum += 1.0/sc.nextInt();
    }
    sum = 1.0/sum;
 
    System.out.print(sum);
  } main {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    double sum = 0;
    for(int i=0; i<N; i++){
      sum += 1.0/sc.nextInt();
    }
    sum = 1.0/sum;
 
    System.out.print(sum);
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc double sum = 0; sum 0 for(int i=0; i<N; i++){
      sum += 1.0/sc.nextInt();
    } int i=0; int i=0; i 0 i<N i N i++ i++ i {
      sum += 1.0/sc.nextInt();
    } {
      sum += 1.0/sc.nextInt();
    } sum += 1.0/sc.nextInt() sum 1.0/sc.nextInt() 1.0 sc.nextInt() sc.nextInt sc sum = 1.0/sum sum 1.0/sum 1.0 sum System.out.print(sum) System.out.print System.out System System.out sum String[] args args
import java.util.Scanner;
class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    double sum = 0;
    for(int i=0; i<N; i++){
      sum += 1.0/sc.nextInt();
    }
    sum = 1.0/sum;
 
    System.out.print(sum);
  }
}
