hmm.

in general, you will probably find that "m + n < 0" is a much more useful phrasing than is "m < -n".

if you don't see why, then consider a test case, such as m = -5, n = 6.
with the former version, it's easy to see that the statement is false; because the size (i.e., absolute value) of n is larger, it's easy to see that adding m + n will create a positive result.

with the latter version, it's much more difficult to verify the falsity of the given inequality, because not only do you have to flip the sign of the '6' but you also have to evaluate an inequality involving two negative numbers, which is not an easy thing to do.

in general, though, you should go with whichever rephrase is most convenient for you. if you are one of the select few who prefer "m < -n" to "m + n is negative", then go for it.