Addition: Appendix+Appendix = Sum.
1, recalculate the position where the addend can be exchanged;
2. Use and subtract one of the addends and the other addend that should be added to make it correct.
Subtraction: subtraction-subtraction = difference
1, the minuend minus the difference should be equal to the minuend;
2. subtraction+difference should be equal to the minuend.