Help with Iif on Report

I can't seem to see what's wrong with my logic, although I am sure it's quite simple:

I have a report showing students, with their year of attendance under each term, 1-4. Some of them attended the Winter term, some Spring, and some both. I need an unduplicated count of students, regardless of year.

I had been using this:
=Sum(IIf([2012 w]>=1,1,IIf([2013 s]>=1,1,0)))

In other words, if this student attended this particular winter term, count him as 1, otherwise, if he attended Spring...

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