Is it possible to find two numbers such that if you add up the sum and the difference of these two numbers, you get $$999$$? If yes, please write it out. If not, please give your reason. 
Overseas Competition->Knowledge Point->Number Theory Modules->Odd and Even Numbers
For example you have number A and B.  A+B = C  A-B = D  (A+B)+(A-B)=2A  Thus, C+D must always be even number. Cannot be odd number. 