In Vijeo Designer, by default, if you do any math calculations in the variable field it will deactivate the "Enable Input Mode".
The solution would be:

1) Create a numeric display with input mode enabled. Let s call this NumericDisplay01. Assign it to INT01 variable for example.

2) Create another numeric display (NumericDisplay02) but with input mode disabled. For this one you would enter the expression (e.g. INT01*2) in the Variable field.

3) Overlay NumericDisplay02 on NumericDisplay01.

In Runtime, touch input will automatically go to NumericDisplay01 since it is disabled for NumericDisplay02.