Web development

I need a new website. I already have a design, I just need you to build a website for my small business. I have a project which will help my business with our numbers. It is not a difficult process that will need to be created. It requires a very large number (1,000 digits or more) to be subtracted from using a number that is approximately 10 digits long, then the new subtotal must be subtracted from using that same 10 digit number, then that subtotal subtracted from with the same 10 digit number, and continuing until there is only a remainder left. We must then have the process reversible by adding the same 10 digit number back again and again in order to get the same very large number that was started with. We must know how many times these subtractions happen and what the exact remainder is. Multiplication and division is not usable in this situation. We want the process to be reversible so we can doublecheck that the number of additions is exactly the same as subtractions. The accuracy of the math is very important, and the speed is also important. I will be including a link to 2 different calculators, one is written in JavaScript and the other is in C. I need you to determine which is more accurate as well as fast. I would like to have this in a website, but this will be proprietary information so the process cannot be seen by the website's users, even those person trying to hack into the website.