Find the sum: $(-39) + (-37) + \cdots + (-1)$.
I notice that this is an arithmetic series, where each term is 2 more than the previous one. I also notice that there are 20 terms in the series, since $-39 = -1 - 2 \cdot 19$. To find the sum of an arithmetic series, I can use the formula $S_n = \frac{n}{2}(a_1 + a_n)$, where $S_n$ is the sum of the first $n$ terms, $a_1$ is the first term, and $a_n$ is the nth term. In this case, $n = 20$, $a_1 = -39$, and $a_n = -1$. Plugging these values into the formula, I get $S_{20} = \frac{20}{2}(-39 + -1) = -400$. 