Find the value of 1 - 2 - 3 + 2 - 3 - 4 + 3 - 4 - 5 + ... + 100 terms .?

The first 100 terms of this series is
(1 - 2 - 3) + (2 - 3 - 4) + ... + (33 - 34 - 35) + 34
The first 33 terms of the above series will given an AP as
-4, -5, -6, ... -36
∴ Answer = 33 x (-20 ) + 34 = - 626