Challenger Puzzle #1

As an educator, I have always enjoyed creating my own puzzles.  This makes it so people cannot just look up answers online.  They either solve it or will never know the answer.  Enjoy this puzzle.  I would call this "extreme" in terms of difficulty.  Good luck!

I have invented a mathematical system.  I can always give you more information about my mathematical system, but this should be enough for you to dig in and figure things out!

I.  10 + 110 = 120

II.  1 + 1 = 10

III.  211 * 10 = 1100

IV.  4020 / 1001 = 20

V.  121,310 - 112,220 = 4020

VI.  43,011 - 24,301 = 13,110

Using these equations, solve the following problem:

Let A = 1.  Let B = A + 1.  Let C = B + 1, and so forth.

What is ABCDEFGHIJK - 1?

Hint:  I used Roman numerals just to differentiate from my math system.  They are not part of the problem.