I'm trying to find the number of terms in the arithmetic series $\displaystyle -4, 2, 8, 14, ....$ which makes the sum strictly greater than $\displaystyle 18,000$.

I know that $\displaystyle S_{n} = \frac{1}{2}\left[{2a+(n-1)d}\right]$, but I somehow can't apply it to this problem.