input: def count_special_chars(word: str) -> int:
      """
      You are given a string word. A letter is called special if it appears both in lowercase and uppercase in word.
  
      Return the number of special letters in word.
  
      Examples:
      >>> word = "aaAbcBC"
      3
      >>> word = "abc"
      0
      >>> word = "abBCab"
      1
  
      Constraints:
      1 <= word.length <= 50
      word consists of only lowercase and uppercase English letters.
      """
  
output:     special_chars = set()
      for char in word:
          if char.islower() and char.upper() in word:
              special_chars.add(char.upper())
          elif char.isupper() and char.lower() in word:
              special_chars.add(char)
      return len(special_chars)