A four-digit number abcd, write it upside down to get a new number dcba. If the original number minus the new number is equal to e997, what number does e represent?
一个四位数abcd，把它倒过来写，得到一个新数dcba．如果，原数减去新数等于e997，问e是代表甚么数？
<result>2</result>