How to add a constraint on an extention in an Use Case Diagram

I have the following problem. :?

I have two use cases. One of them extends the other. Furthermore I have two actors. Both use the main use case. One of them additionally use the extended use case.

How can I add a constraint on the extend relation, so that the extended use case just can run provided that the contraint is complied?

e.g. use case2 >extend< use case1 - if…the actor has the permission to do that