Is IM correct in roman numerals?

Answer 1

IM is not correct even though M-I=999

Answer 2

not to equal 999.

Another Answer:-

The Romans themselves would probably approve of it because they would have calculated 999 on a abacus counting device as DCCCCLXXXXVIIII and then simplified it to IM in written form.

But under today's rules governing the Roman numeral system introduced during the Middle Ages 999 is supposed to be CMXCIX.

The former seems to be more plausible than the latter.