已知矩阵$A=\left[\begin{array}{cc}-1& 2\\ 1& x\end{array}\right],B=\left[\begin{array}{cc}1& 1\\ 2& -1\end{array}\right]$，向量$a=\left[\begin{array}{c}2\\ y\end{array}\right]$，$x,y$是实数，若$A\mathit{a}=B\mathit{b}$，求$x+y$的值.