(n^2 + n)/2 will come in handy; there are other ways to solve this too.

S_n = n(2a_1+(n-1)d)/2

a_1 is the first term; d is the difference between the two terms

Sn=200(2*2+(199)2)/2

= 40,200

OR:

(n^2 + 200)/2 = 20,100, but you want the even numbers;

Thus, multiply by 2 = 40,200