The HyperCaesar cipher is just like the Caesar cipher, except that the key for each step is the initial key + the sum of the previously encoded characters. For example, if the initial key is 3 and the text to be encoded is "eve", then the first "e" is encoded with a key of 3. The next letter is encoded with a key of 104 because the character "e" has Unicode value 101. The third letter is encoded with a key of 222 becase the character "v" has a code value of 118 and 222 = 3 + 101 + 118. The summation process is carried out modulo 256.
(Despite its seeming complexity, this is not a safe cipher to use for your secret correspondence. It is easy for anyone wishing to decode your messages to try all 256 possible keys.)
Your task is to implement the decryption method.