input: def count_pairs(numbers: List[int], x: int) -> int:
      """ Given a list of integers and a target sum, count how many pairs of integers
      in the list sum up to the target sum.
      >>> count_pairs([1, 3, 6, 2, 2, 9, 1], 4)
      2
      >>> count_pairs([1, 5, 3, 7, 9], 10)
      1
      """
output:     count = 0
      seen = set()
      for num in numbers:
          complement = x - num
          if complement in seen:
              count += 1
          seen.add(num)
      return count