decrypting a string

[Carlo Gambino]

I am looking into learning the basics of cryptography and would like to use python to help me overcome this small challenge. I would like to create a program that takes a user input string and decrypts it.

The encrpytion is super simple. The string

aaaaa

would return

abcde

I was thinking I could assign a value to each character and increment the value accordingly, but I was looking for a way to avoid doing this manually. Is there an easier way to accomplish such a thing?