input: 
  from typing import List
  
  
  def extract_words(word: str, letter: str) -> List[str]:
      """
      Extract all the words starting with a specific letter from a given string.
      
      >>> extract_words('This is a sample sentence to test the program', 's')
      ['sample', 'sentence', 'to', 'the']
      >>> extract_words('Python programming is fun and exciting', 'p')
      ['Python', 'programming']
      """
      words_list = []
      for w in word.split():
          if w[0].lower() == letter.lower():
              words_list.append(w)
      return words_list
  
  if __name__ == '__main__':
      words = 'This is a sample sentence to test the program'
      letter = 's'
      print(extract_words(words, letter))
  
output:     words_list = []
      for w in word.split():
          if w[0].lower() == letter.lower():
              words_list.append(w)
      return words_list