Find the closed form for:

$\displaystyle \sum_{i=-1}^{n}i$

It's given that the closed form $\displaystyle \sum_{i=1}^{n}i = (n(n+1))/2$ so we can write:

$\displaystyle \sum_{i=-1}^{n}i = (-1) + 0 + \sum_{i=1}^{n}i = (n(n+1))/2$

...

$\displaystyle ( (n+2)(n-1) )/2$

I understand the final closed form answer but could someone explain where the terms (-1) + 0 come from on the first line. I'm assuming the -1 comes from the lower bounds of the summation..