Data Validation Formula with 2 Letter Option

Good Morning,

I have this formula in Data validation, however now they want to change it so the user can enter "E" or "C" How can I change the current formula below to take either E or C? as it is now it only takes "E"

Here is what I have now.

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Thanks for the help,

Mike

Perhaps:

=LEN(SUBSTITUTE(SUBSTITUTE(UPPER(B5),"E",""),"C",""))=0