input: def count_pairs(numbers: List[int], x: int) -> int:
      """ Count the number of pairs in the list 'numbers' that sum up to 'x'.
      The list 'numbers' is assumed to be sorted in ascending order.
      
      >>> count_pairs([1, 3, 6, 7, 9], 10)
      2
      >>> count_pairs([0, 1, 2, 3, 4], 5)
      2
      """
output:     count = 0
      left, right = 0, len(numbers) - 1
      
      while left < right:
          if numbers[left] + numbers[right] == x:
              count += 1
              left += 1
              right -= 1
          elif numbers[left] + numbers[right] < x:
              left += 1
          else:
              right -= 1
      
      return count