\[a_{2} = - 1;\ \ a_{3} = 1;\ \]
\[d = a_{3} - a_{1} = 1 + 1 = 2;\ \]
\[a_{1} = - 1 - 2 = - 3;\]
\[S_{20} = \frac{2a_{1} + d(n - 1)}{2} \cdot n =\]
\[= \frac{- 6 + 2 \cdot 19}{2} \cdot 20 =\]
\[= 32 \cdot 10 = 320.\]
\[Ответ:320.\]