input: def count_pairs(numbers, x):
      """ Count the number of pairs in given list 'numbers' that sum up to 'x'.
      >>> count_pairs([1, 3, 6, 8, 5, 7, 4], 10)
      2
      >>> count_pairs([1, 9, 5, 0, 20, -4, 12, 16, 7], 12)
      3
      """
output:     count = 0
      seen = set()
      for num in numbers:
          complement = x - num
          if complement in seen:
              count += 1
          seen.add(num)
      return count