Open the executable in hydra and examine the code.


You can see that when buying flags, number_of_flags and account balance vars is used, This vars are <uint> type, so we can try to buy an unrealistic number of flags for an amount greater than 4,294,967,295 (2^32 is max value of uint).

When account_balance variable overflows, it becomes negative. Finally, we buy a large number of flags for a negative price and get money to buy a second flag. A more detailed process is described here https://en.wikipedia.org/wiki/Integer_overflow

In my case, I bought 100,000,000 (100kk*1000 > 4.2kkk) flags and received enough money on my account to buy second flag :)