input: Write a function that takes a list of integers `numbers` and a target sum `x`, and counts the number of pairs of elements in the list that add up to `x`. The function should return the count of such pairs. If the list is empty or contains less than two elements, the function should return 0.
  
  ### Function Definition:
  def count_pairs(numbers, x):
      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
  
  ### Test Cases:
  assert count_pairs([1, 9, 2, 8, 3, 7, 4, 6, 5, 5], 10) == 5
  assert count_pairs([1, 1, 1, 1], 2) == 2
  assert count_pairs([10, 20, 30, 40], 50) == 2
  assert count_pairs([], 10) == 0
  assert count_pairs([1], 10) == 0
output: def count_pairs(numbers, x):
      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