Arithmetic addition; joining, not adding

Very simple problem, although unfortunatly the solution is beyond me.

I am trying to add two variables, but my AS is joining them

volume1=function(answer1){
aa=wtft12;
aaa=wtin;
bb=htft
12;
bbb=htin;
c=dtin;
a = aa + aaa;
b = bb + bbb;
result1=a;
return result1;
};

There is more to this script, but I cut it down to this in order to focus on the error, which results in a joining aa and aaa. i.e. aa=36 aaa=4 results in a=364, where what I want would be a=40

Hope that is a clear explaination. Any takers?