in how many ways one can choose the number

Question

Let $n, m \in N \cup 0$, $-n\leq m\leq -n$ let $x_i \in R^n, i=1, \ldots, n$. Let $y_i\in\{-1,1\}, \, i=1, \ldots, n$. In how many possibilities one can choose $x_i$ and $y_i$ such that $\sum_{i=1}^nx_iy_i=m$?