Cumulative Variable?

I want to create a variable - Cumulo - which adds the value of another parameter - Adder(which animates) - to itself on every frame.

So on:
frame 1 - Adder = 10 - Cumulo = 10
frame 2 - Adder = 10 - Cumulo = 20
frame 3 - Adder = 10 - Cumulo = 30
frame 4 - Adder = 20 - Cumulo = 50
frame 5 - Adder = 20 - Cumulo = 70

How do I write an expression to do that?
My final values will be much less user friendly to think about.

Thanks