arcgis 10.1 - Arcpy how to conduct addition in a script, function needed?

I'm trying to work on my first arcpy script and am running into some troubles. The objective is to create a new field, check several existing fields (Cnt1,2,3 and Pop1,2,3) for missing data (-99 value) and if find missing data populate new field with a 0, if not then populate the field with the result from a simple calculation. From a separate related post I got some feedback that the problem was that I am concatenating the values, not adding them and this if-elif statement might need to be within a function and then called. Any advice greatly appreciated!