input: 
  from typing import List
  
  
  def combine_extensions(class_name: str, extensions: List[str]) -> str:
      """
      Combine the class name with the strongest extension based on the criteria:
      The strongest extension is determined by the difference in the counts of uppercase
      and lowercase letters in the extension's name. The extension with the highest negative
      difference is the strongest. If there are multiple extensions with the same strength,
      the first one in the list is chosen.
  
      >>> combine_extensions('my_class', ['AA', 'Be', 'CC'])
      'my_class.AA'
      >>> combine_extensions('Slices', ['SErviNGSliCes', 'Cheese', 'StuFfed'])
      'Slices.SErviNGSliCes'
      """
  
output:     strongest_extension = None
      max_difference = float('-inf')
      
      for ext in extensions:
          upper_count = sum(1 for c in ext if c.isupper())
          lower_count = sum(1 for c in ext if c.islower())
          difference = lower_count - upper_count
          
          if difference > max_difference:
              max_difference = difference
              strongest_extension = ext
      
      return f"{class_name}.{strongest_extension}"