After a four-digit number is multiplied by 9, the product obtained is also a four-digit number, and the digits of the product are in the opposite order of the original four-digit number, so what is the original four-digit number?
一个四位数乘9后，得到的积也是四位数，而且积的各位数字与原四位数的数字顺序正好相反，那么原来的四位数是多少．
<result>1_089</result>