Accumulator

I found this program (source) that lets me add positive numbers to a variable, but it won't give me a flag unless that variable is negative! Can you help me out? Navigate to /challenge/accumulator64 on the shell server to try your exploit out!

Note: Please use accumulator64_patched for running the challenge and getting pwn.college flag. accumulator64 binary is original source and accumulator64_patched is made for it to work with pwn.college.