Here's an explanation for those who don't want to spend time thinking about it:
A , is put before last group of digits. If lastComma = n, then indexes of the characters up to the comma are 0, 1, ..., n and the last digits have indexes n+1, n+2, n+3, therefore amount.length() should equal n+4, but if digits are missing, it's smaller, so amount.length() = n+1+visibleDigits, from where we can get visibleDigits = amount.length() - lastComma (n) - 1, and 3 - visibleDigits is how many zeroes we have to add. I tested it in a real contest

and it gave correct results. the (detected line: ) in this image shows the amount variable