input: from typing import List, Optional
  
  
  def find_missing_number(numbers: List[int]) -> Optional[int]:
      """ Given a list of integers in ascending order with one missing number,
      find and return the missing number. If no number is missing, return None.
      >>> find_missing_number([1, 2, 3, 4, 6])
      5
      >>> find_missing_number([10, 11, 12, 14, 15])
      13
      >>> find_missing_number([1, 2, 3, 4, 5])
      None
      """
output:     n = len(numbers) + 1
      expected_sum = n * (n + 1) // 2
      actual_sum = sum(numbers)
      missing_number = expected_sum - actual_sum
      if missing_number == n:
          return None
      return missing_number