Could you provide a little more info on how it breaks and what control you are using? There are a couple things I can mention off the top of my head that should point you in the right direction.
First, I would use a RadMaskedNumericInput
control. Next, I would make sure that your logic can accept a negative value. Simply putting a "-" symbol in front of a number won't automatically tell your logic that it is a less-than-zero value until you cast it into an integer properly.
For instance, to make sure that your value is correct, you can use a regex to strip out the character "-", and then apply this to make it a negative integer.
myInt = myInt * -1;
This may be a little overboard or I am making it more complicated than it needs to be, but you can just create a method that will handle all
the cells in your spreadsheet.
One last thing, do you have to make your own spreadsheet? If not, then you can use RadGridView and accomplish the same functionality as an Excel program.