Need help adding two cells until one reaches a certain number

Hi All,

I have done some extensive searching for a formula to help with my dilemma but to no avail.
Given I have found several answers on this board it seems to be the best place to ask around.

I have two cells adding into a third, I would like them to keep adding until one of the cells reaches a certain number then for the third to stop adding.

Ex. B1 = A1 + A2, until A1 reaches 12 then B1 stops taking from A1.
A1: 5, is changed to 7, is changed to 12, is changed to 15
A2: 5
B1: 10, would be changed to 12, would be changed to 17, would stay at 17


Any help will be very appreciated.

Thank you,

Brilliant!
Thank you very much, Benishiryo.