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