input: 
  from typing import List
  
  def extract_words(words: 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']
      """
      words_list = []
      for w in words.split():
          if w[0].lower() == letter.lower():
              words_list.append(w)
      return words_list
  
output:     words_list = []
      for w in words.split():
          if w[0].lower() == letter.lower():
              words_list.append(w)
      return words_list