How do I use the pumping lemma for a^n b^m a^(n+m) ? How can I choose the pumping length?

$ L = {a^n + b^m + a^{n+m}}$

This is the language I want to show is not regular.

Now my problem is to choose p correctly.

Can I just set it as p=2*(n+m) ?

That’s the problem I am facing now.

Thanks for any help I am starting to learn it to use the pumping lemma.