can't figure out sql query

[Robert Brenstein]

I am somehow dense today and can't figure out an SQL query :(

table 'users' has each user identified by 'userid'
table 'students' has references to 'users' in column 'studentid'
table 'teachers' has references to 'users' in column 'teacherid'

Yes, I am using the traditional keys.

I am trying to find a count of users who are neither teachers nor 
students, that is to count records in 'users' that have no entries in 
tables 'students' and 'teachers', and I am drawing a blank. Note that 
'students' and 'teachers' may have multiple entries for a given user 
since these tables establish M:M relations between 'users' and 
'courses'.

Robert