Team Velocity changes if backend team not involved?

Hi All, 

I have a tricky question, 

For example if my current cross functional team capacity 50 points.

And during the sprint planning we found that all the stories and tasks will belong to the front end team only. 

How many points we should take? Should it still the same and be 50 points and should be less? And how to calculate it of we are going to change it?

Thank you