Take X Characters From One Cell And Y Characters From The Next?

So we have people's names in a table. First name in one column and last name in another column. We have a 3rd column where we can use 8 characters to do a combination of First 5 of Last Name + first 3 of first name. However, if someone's last name is only 3 or 4 characters, we'd then want to take more from the first name to fill out the 8 characters.

So:
Charles Johnson -> Johnscha
John Smith -> Smithjoh
Willian Wu -> Wuwillia

What's the best way to do that without creating some crazy formula with tons of if/thens?