Adding Negative Numbers in c#

HI,
how to add a negative numbers in c#.
ex. one textbox1 have a 10 value and another textbox2, i am entering -2 value. but when i enter "-" sign or -2 the error is occur. ie. inpute string was not in correct format. all this is done on textbox2_TextChanged event. i am using Asp.net 2.0 (Window Application) and C#.