Output a string's cumulative slope Challenge

Given a string such as Hello World!, break it down into its character values: 72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100, 33. 

Then calculate the difference between each consecutive pair of characters: 29, 7, 0, 3, -79, 55, 24, 3, -6, -8, -67.

Finally, sum them and print the final result: -39.

Rules


Standard loopholes apply
No using pre-made functions that perform this exact task
Creative solutions encouraged
Have fun
This is marked as code-golf, the shortest answer in bytes wins but will not be selected.
Python, 29 bytes



lambda s:ord(s[-1])-ord(s[0])


The sum of the differences forms a telescopic series, so most summands cancel out and
(s1 - s0) + (s2 - s1) + &hellip; + (sn-1 - sn-2) + (sn - sn-1) = sn - s0.

If taking a byte string as input is allowed

lambda s:s[-1]-s[0]


will work as well for 19 bytes.

Test both on Ideone.