NXT calculator

This NXT project doesn't need complicated build or a lot of parts of Lego set. Only NXT controller brick for the environment, but this can be an advance programming level using NXT-G software.
The calculator program shown on NXT brick :

Well...... you may know how many variable that you needs.
And then how to input number?????
1. At the first time running program, all the number must be start from 0 (zero).
and the first position on number 1, increase the number using NXT right button, and decrease using NXT left button, wait until Enter/NXT enter button pressed.
2. When entered then position move to operator, there are 4 operator must be added, addition,substraction,multiplication, and division (using right and left NXT button as navigation) Then wait for Enter.
3. Move to number 2, as same as number 1 case, start from zero and we need right and left button for increase and decrease the number.Then wait for Enter.
4. When entered, the result must be shown on the NXT brick.

Example :

5 x 6 = 30

Good luck..........