Not entirely sure how to classify this problem in terms of specific areas of Statistics, so sorry for the vague title. I've been staring at this for about half an hour to no avail. The problem goes...

*A ***four character** password is generated by choosing each character randomly from **26 letters** (a,b,c,...,z) and **ten digits** (1,2,...,9). No password can contain only digits, and no password can consist of the same character repeated four times. **How many valid passwords are there?**

Any ideas? I'm sure it's not that complicated but it's hurting my head