Is that all the codes that you have? Did you ever set any value to your variable 'commonVar1' before calling GetcommonVar1() method? If not, I suppose the result would be unexpected and the -1 is only a coincidence.
If I'm not wrong, the child's property 'commonVar1' takes precedence over the parent's. Anyway as a side note, I don't see any reason why you should declare a variable in the child class having the same name as the parent's.