Java: Code won't run after 21 inputs in a while(true) loop after appending an ArrayList

We have a High School activity where we utilize ArrayList methods to create a mean and median program. However, I encounter a problem when I inputted 20 data as shown below. All answers and suggestions are appreciated.

THE PROBLEM:

Create a MeanMedian2 class so that the user can enter any number of values up to 20. If the list has an even number of values, the median is the numeric average of the values in the two middle positions. Allow the user to enter 9999 to quit entering numbers.

//https://www.softwaretestinghelp.com/java-arraylist-tutorial/
//https://stackoverflow.com/questions/16242733/sum-all-the-elements-java-arraylist
//https://stackoverflow.com/questions/16242733/sum-all-the-elements-java-arraylist
//https://stackoverflow.com/questions/22023053/how-do-i-print-a-single-item-on-a-array-list

import java.util.*;
class MeanMedian2{

   public static void main(String[] args){
      Scanner input = new Scanner(System.in);
      ArrayList<Integer> numbers = new ArrayList<>();

      int reference = 0; 
      int count = 1;
      int ender = 9999;
      
      while(true){
          System.out.println("Enter number " + count + ":" );
          int num = input.nextInt();
          if (num == ender){
              break;
          }else{
              numbers.add(num);
              reference += 1;
              count += 1;
          }
      }

      //Mean
      double sum = 0;
      for (int i = 0; i < numbers.size(); i++)
           sum += numbers.get(i);
      double mean = sum/numbers.size();
      
      //Median
      int median;
      Collections.sort(numbers);
      int length = numbers.size();
      int oddOrEven = length % 2;
      int Odd = (length + 1)/2;

      if (oddOrEven != 0 ){ 
          median = numbers.get(Odd - 1);
      }else{
          int a = numbers.get((length/2)-1);
          int b = numbers.get(length/2);
          median = (a+b)/2;

      }
      
      

      //Final
      System.out.print("You entered: ");
      for(int i = 0; i < numbers.size() - 1; i++){
          System.out.print(numbers.get(i) + ", ");
      }
      System.out.println(numbers.get(length-1));
      System.out.println("The mean is " + mean + " and the median is " + median );
      

   }
}

Instance 1:

/**
Enter number 1:
25
Enter number 2:
50
Enter number 3:
500
Enter number 4:
550
Enter number 5:
450
Enter number 6:
600
Enter number 7:
200
Enter number 8:
10
Enter number 9:
700
Enter number 10:
9999
You entered: 10, 25, 50, 200, 450, 500, 550, 600, 700
The mean is 342.77777777777777 and the median is 450**/

Instance 2. I inputted 20 data and 1 9999 to stop (21 in all)

/** 
javac MeanMedian2.java
java MeanMedian2Exception in thread "main" 
java.util.InputMismatchException
     at java.util.Scanner.throwFor(Scanner.java:864)
     at java.util.Scanner.next(Scanner.java:1485)
     at java.util.Scanner.nextInt(Scanner.java:2117)
     at java.util.Scanner.nextInt(Scanner.java:2076)
     at MeanMedian2.main(MeanMedian2.java:19)**/