for an application

 i'm using a textbox and a button in form.i need to perform sum of 2 no's.i'll enter first one in textbox and click button now the content in the textbox has to be erased and i have to enter another value and then i should get the answer.but the prob is when i click the button the code inside gets executed without waiting for the second number.i think u folks got my point.
how to achieve this

 
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