input: from typing import List
  
  def count_pairs(numbers: List[int], x: int) -> int:
      """ Count the number of pairs in the list `numbers` that sum up to `x`.
      >>> count_pairs([1, 9, 2, 8, 3, 7, 4, 6, 5, 5], 10)
      5
      >>> count_pairs([1, 2, 3, 4, 5, 6, 7, 8, 9], 10)
      4
      """
output:     count = 0
      seen = set()
      for num in numbers:
          complement = x - num
          if complement in seen:
              count += 1
          seen.add(num)
      return count