Calculate Money in Bank

Hercy wants to save money in the bank. Initially, he has a certain amount of money that he wants to deposit in the bank. He will deposit this amount every day for a certain number of days.

The bank's rules for calculating interest are as follows:

If it's the ith day (1-indexed) and Hercy has money in the bank, he will receive (i-1) dollars of interest on that day. If it's the ith day and Hercy has no money in the bank, he will not receive any interest on that day. Calculate the total amount of money Hercy will have in the bank after the given number of days n.