What is the difference between the sum of the first 400 even counting numbers and the sum of the first 400 odd counting numbers?

The sum of the first n even integers is given by  (n)(n + 1)

The sum of the first n odd integers is given by n^2

So we have

(400)(401) - (400^2)   =

(400)(400 + 1)  - (400^2)  =

400^2 + 400 - 400^2   = 400