While we are more than willing to help those that are stuck, that doesn't mean that we are here to do it all for you! We can't do all the work, you are either getting paid for this, or it's part of your grades and it wouldn't be at all fair for us to do it all for you.
So we need you to do the work, and we will help you when you get stuck. That doesn't mean we will give you a step by step solution you can hand in!
Start by explaining where you are at the moment, and what the next step in the process is. Then tell us what you have tried to get that next step working, and what happened when you did.
But to be honest, if you think expanding
num
to a long - 64 bit - integer is a good idea, I just hope you don't want to use your computer for some significant number of years, as that loop is going to take considerable time to complete ...
What I would suggest you do is scrap the code you have as it isn't at all relevant to the task you have been set, and start again from scratch.
If you are having problems getting started at all, then this may help:
How to Write Code to Solve a Problem, A Beginner's Guide[
^]