Let $A=\left[\begin{array}{ll}a & b \\ c & d\end{array}\right]$ and $B=\left[\begin{array}{l}\alpha \\ \beta\end{array}\right] \neq\left[\begin{array}{l}0 \\ 0\end{array}\right]$ such that $A B=B$ and $a+b=2021$, then the value of $a d-b c$ is equal to $\_\_\_\_$ .
