Using contact post-processing variable

Hello everyone,

I want to use the post-processing variable solid.gap_p1 from the Contact pair definition in defining a gap-dependent force, e.g. force = C *gap_p1.

How can I choose the gap variable in the definition of my contact force? When I simply enter solid.gap_p1
in the Global definitions -> Variables it turns red.

Can you help me out?

Thanks,
Gianna