20. For a certain commodity, if you buy 100 (including 100) pieces or less, it is settled at the retail price; if you buy 101 (including 101) pieces or more, it is settled at the wholesale price. It is known that the wholesale price is 2 yuan lower per piece than the retail price. A person originally intended to buy a certain number of pieces of this commodity, and needed to pay $a$ yuan at the retail price, but if they bought 21 more pieces, they could settle at the wholesale price, which would also be exactly $a$ yuan ($a$ is an integer), then the value of $a$ is $\qquad$
$\begin{array}{l}20 \\ 840\end{array}$