Adding the amount from one textbox to another in vb.net

i've created 2 textboxes in vb.net the first one i put the number "1" then i click on calculate button it calculates the cost of the item i've selected, then i clear the item and the "1" is deleted from the first textbox.

iam show the number "1" from the first textbox in the second textbox iam able to do this, but when i clear the first textbox, i want to add the number "1" that is stored in the second textbox with the new number eg "2" that i calculate in the first text box after i cleared the first one. eg the result of the second textbox would be "3".

if anyone can help me with this please let me know it is for a test.