jmc

The sum is an arithmetic series with common difference 2. Let $n$ be the number of terms. Then the $n$th term is $-1$, so $-39+(n-1)(2)=-1$, or $n=20$. The sum of an arithmetic series is equal to the average of the first and last term, multiplied by the number of terms, so the sum is $[(-39)+(-1)]/2\cdot 20=\boxed{-400}$.