We start by opening the website.
We see that there is some kind of trading platform where we can exchange money between different currencies. Unfortunately we can't change more currency than we have.
But exchanging a negative amount of money is possible.
This grants us some money but also gives us a negative balance.
Unfortuantely we can't Join the Glacier Club (which is probably the flag) without emptying our balance.
After some testing, we figure out that exhanging extremely large amounts will result in rounding errors.
So we start by exchanging
Then we can subtract
Now we erase the negative balance by exchanging
Then we can join the Glacier Club and get the flag.