input: Write a function to search for the index of a specific word in a text, but only if the word's first letter matches the given letter.
  def search_word_index(text, word, first_letter):
      index = -1
      for i, w in enumerate(text.split()):
          if w.startswith(first_letter) and w == word:
              index = i
              break
      return index
  
  assert search_word_index('This is a sample sentence to test the program', 'sample', 's')==3
  assert search_word_index('This is a sample sentence to test the program', 'test', 't')==5
  assert search_word_index('This is a sample sentence to test the program', 'program', 'p')==7
  assert search_word_index('This is a sample sentence to test the program', 'nonexistent', 'n')==-1
output: def search_word_index(text, word, first_letter):
      index = -1
      for i, w in enumerate(text.split()):
          if w.startswith(first_letter) and w == word:
              index = i
              break
      return index