Getting cumulative values from motion in expresso

Hi everyone,

Good to finally be posting here after months of reading!
Here's my question:

Through Expresso I have linked the motion of a sphere (slave) to the y-position of another sphere (master) which oscillates up and down. Rather than the slave moving back and forth, as it does at the moment, I'd like the master's motion in the y-direction to not only be cumulative but always positive (modulated I guess), so that it outputs an ever increasing value that is fed to the slave sphere.

I'd be really grateful if someone could explain how I might achieve this, as I've tried a lot of methods that seem to make sense mathematically but I can't get them to work in Expresso. The other problem is that I don't know the correct names for the things I'm looking for, so hopefully someone who does will find this simpler!

Cheers!

Dan Watson