Ez Question Adding Numbers

[VB.NET]

How do i do that when Button5 is pressed it will add 3600 to the textbox1 and if button4 will add 1800 cuz when i do press the button 5 textbox says 3600 its ok but when i press button4 the 3600 change to 1800 i want it to add like 3600 + 1800 so if both are clicked it will be 5400 or...