Next comes adding them, then a binary to decimal converter. You will soon be making a calculator. I made my first decimal to binary converter last week, and just finished a binary calc. The binary to decimal is easy... Just takes time to understand. I can explain if you wish.
If a spark goes past an output that corresponds to a number, and it isn't the right number, it will past PTCT and all sparks coming from the side will not reach the display. Basicly, it deactivates all other posibilities and allows the correct one... Play it over a few times.
Do you need any other help. Just a warning: It will be very time consuming. I finished mine in a couple days, then it didn't work. So I spent anout a week getting it right.