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