input: Write a function to count pairs of numbers that sum to a specific value.
  assert count_pairs([1, 2, 3, 4, 5, 6], 7) == 2
  assert count_pairs([1, 2, 3, 4, 5, 6], 8) == 2
  assert count_pairs([1, 2, 3, 4, 5, 6], 10) == 1
output: def count_pairs(numbers, target):
      count = 0
      seen = set()
      for number in numbers:
          complement = target - number
          if complement in seen:
              count += 1
          seen.add(number)
      return count