import java.util.Scanner;
public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    int [] Ai = new int[N];
    double [] Gi = new double[N];
    double sum = 0;
    for(int i=0;i<N;i++){
      Ai[i] = scan.nextInt();
      Gi[i] = 1.0/Ai[i];
      sum += Gi[i];
    }
    //System.out.println(sum);
    double sum2 = 1.0/sum;
    System.out.println(sum2);
  }
} import java.util.Scanner; Scanner java.util public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    int [] Ai = new int[N];
    double [] Gi = new double[N];
    double sum = 0;
    for(int i=0;i<N;i++){
      Ai[i] = scan.nextInt();
      Gi[i] = 1.0/Ai[i];
      sum += Gi[i];
    }
    //System.out.println(sum);
    double sum2 = 1.0/sum;
    System.out.println(sum2);
  }
} Main public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    int [] Ai = new int[N];
    double [] Gi = new double[N];
    double sum = 0;
    for(int i=0;i<N;i++){
      Ai[i] = scan.nextInt();
      Gi[i] = 1.0/Ai[i];
      sum += Gi[i];
    }
    //System.out.println(sum);
    double sum2 = 1.0/sum;
    System.out.println(sum2);
  } main {
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    int [] Ai = new int[N];
    double [] Gi = new double[N];
    double sum = 0;
    for(int i=0;i<N;i++){
      Ai[i] = scan.nextInt();
      Gi[i] = 1.0/Ai[i];
      sum += Gi[i];
    }
    //System.out.println(sum);
    double sum2 = 1.0/sum;
    System.out.println(sum2);
  } Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int N = scan.nextInt(); N scan.nextInt() scan.nextInt scan int [] Ai = new int[N]; Ai new int[N] N double [] Gi = new double[N]; Gi new double[N] N double sum = 0; sum 0 for(int i=0;i<N;i++){
      Ai[i] = scan.nextInt();
      Gi[i] = 1.0/Ai[i];
      sum += Gi[i];
    } int i=0; int i=0; i 0 i<N i N i++ i++ i {
      Ai[i] = scan.nextInt();
      Gi[i] = 1.0/Ai[i];
      sum += Gi[i];
    } {
      Ai[i] = scan.nextInt();
      Gi[i] = 1.0/Ai[i];
      sum += Gi[i];
    } Ai[i] = scan.nextInt() Ai[i] Ai i scan.nextInt() scan.nextInt scan Gi[i] = 1.0/Ai[i] Gi[i] Gi i 1.0/Ai[i] 1.0 Ai[i] Ai i sum += Gi[i] sum Gi[i] Gi i double sum2 = 1.0/sum; sum2 1.0/sum 1.0 sum System.out.println(sum2) System.out.println System.out System System.out sum2 String[] args args public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    int [] Ai = new int[N];
    double [] Gi = new double[N];
    double sum = 0;
    for(int i=0;i<N;i++){
      Ai[i] = scan.nextInt();
      Gi[i] = 1.0/Ai[i];
      sum += Gi[i];
    }
    //System.out.println(sum);
    double sum2 = 1.0/sum;
    System.out.println(sum2);
  }
} public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    int [] Ai = new int[N];
    double [] Gi = new double[N];
    double sum = 0;
    for(int i=0;i<N;i++){
      Ai[i] = scan.nextInt();
      Gi[i] = 1.0/Ai[i];
      sum += Gi[i];
    }
    //System.out.println(sum);
    double sum2 = 1.0/sum;
    System.out.println(sum2);
  }
} Main
import java.util.Scanner;
public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    int [] Ai = new int[N];
    double [] Gi = new double[N];
    double sum = 0;
    for(int i=0;i<N;i++){
      Ai[i] = scan.nextInt();
      Gi[i] = 1.0/Ai[i];
      sum += Gi[i];
    }
    //System.out.println(sum);
    double sum2 = 1.0/sum;
    System.out.println(sum2);
  }
}
