input: from typing import List, Union
  
  def extract_words(word: str, letter: str) -> Union[List[str], str]:
      """ 
      Extract all the words starting with a specific letter
      >>> extract_words('This is a sample sentence to test the program', 's')
      ['sample', 'sentence', 'to', 'test', 'the', 'program']
      >>> extract_words('This is a test', 'a')
      ['a']
      """
      words_list = []
      for w in word.split():
          if w[0] == letter:
              words_list.append(w)
      return words_list
output:     if not words_list:
          return "No words found"