How to use addition in IF condition in RTF template

My requirement is as follow:

if (a + b) > 0 then (c+d) else (e+f)

I put in the following in the RTF template for the column but I'm not getting the correct result:

<?xdofx: if <?Field 1 + Field 2?> > 0

then <?Field 3 + Field 4>

else <?Field 5 + Field 6>

end if?>

Please can someone tell me what I'm doing wrong and how to fix it.

Thanks