阅读李白的《送友人》一诗，完成小题。
                                                                                                                                   青山横北郭^①，白水绕东城。
                                                                                                                                   此地一为别，孤蓬^②万里征。
                                                                                                                                   浮云游子意，落日故人情。
                                                                                                                                    挥手自兹^③去，萧萧班^④马鸣。
       【注】①郭：城墙外的墙，指城外。②蓬：草名，枯后随风飘荡，这里喻友人。③兹：现在。④班：分别。班马，是离别的马。

.等差数列$\left\{a_n\right\}$满足$a_2=12,a_n=-20,d=-2$则$n=$（  ）