Decrypt message - being interviewer

DecryptMessage_beingInterviewer.py

 def decrypt(word):
  """
  c   r   i   m   e
  99	114	105	109	101  (ascii value)
  100 214 319 428 529  (+1 first letter, add previous from 2nd to last one)
  100 110 111 116 113  (substract 26 from every letter until it is between ascii('a') and ascii('z'))
  
  0     104   208    312    416    
  0*26  4*26  8*26   12*26  16*26
 
  a -> 97
  z -> 122
  
  a -> a + 1
  b -> b + (a + 1)
  c -> c + (b + (a + 1))
  d -> d + (c + (b + (a + 1)))
  
  - add (4*i)*26 from every letter
  - letteri = letteri - letteri-1
  - letter0 = letter0 - 1
  
  given by Julia:
   formula: Sum(source[i]), encrypt[i] = source[i] + sum(source[0 to i -1]) - 26 * n
                                  // source[i] = encrypt[i] - sum(source[0 to i - 1]) + 26 * n
                                  
  """
  if not word:
    return ""
  
  values = [ord(c) for c in word]

  for i in range(len(values) - 1, 0, -1):
    values[i] -= values[i - 1]   # outofrange a - z
    
  values[0] -= 1
    
  for i in range(len(values)):
    while values[i] < ord('a'):
      values[i] += 26
  
  message = "".join([chr(n) for n in values])
  
  return message

word = "dnotq"
r = decrypt(word)
print(r)