You carry on into negatives. Eventually you'll get to -32,768, and then you can start again from 32,767 - assuming you have a 16 bit brain that is. God help you if yours is 32 bit.
You enter negative integer land. Assuming you started at binary twos complement +4 you've only got five counts to decide on register width before 1111 1111 then 1111 1110 etc
Personally I'd start at a bigger number like decimal 127 aka 0111 1111