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