Problem in multplying two columns in gridview in aspnet

I have a gridview for my web application in which there is quantity and unit price column and total column. If I type quantity and unit price then I want to display the total price automatically in next column. But for my current coding it will display only after the page reload by I clicking on the add button only. I need to display the total amount automatically when I move to the Total column. This is my current code.