loop creation in simulink

Hi, I want have two values that are constantly updated in simulink. Now I want to add these two values, and in the next sample time, I want to add these two values to the previous sum. I want to do this for 80 times. Rest the values. And do it again. I want to do this in real time in simulink

[Azzi Abdelmalek]

Your question is not clear, illustrate with variables a , b and the result s