A mail truck transports mail from point A to point B every day, and there are 18 stations along the way (including A and B). When starting from point A, load one mail sent to each of the 17 stations in the back, unload the mails sent to this station by the previous stations after arriving at each station behind, and load one mail sent to the following stations by this station at the same time, Assuming that the number of postal items remaining after the postal car finishes loading and unloading at the nth station is an(n=1, 2,...,18), what is the maximum value of an?
一辆邮车每天从A地往B地运送邮件，沿途(包括A，B)共有18站．从A地出发时，装上发往后面17站的邮件各一个，到达后面各站后卸下前面各站发往该站的邮件，同时装上该站发往下面各站的邮件各一个，设邮车在第n站装卸完毕后剩余的邮件个数为an(n=1，2，…，18)，则an的最大值是多少．
<gadget id="calculator">18 * 9</gadget>
<output>162</output>

<gadget id="calculator">9 ** 2</gadget>
<output>81</output>

<gadget id="calculator">162 - 81</gadget>
<output>81</output>

<result>81</result>