input: def count_pairs(numbers, x):
      """ Count the number of pairs in the list 'numbers' that sum up to 'x'.
      >>> count_pairs([1, 2, 3, 4, 5, 6], 10)
      1
      >>> count_pairs([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 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