Add Str Numbers To Give A Total

tell me the syntax for adding string numbers together to give a total? Code:

strNumber1 = 4
strNumber2 = 2
strTotal = strNumber1 + strNumber2


The above code gives the result 4+2

If I change the strtotal to strNumber1 * strNumber2 it gives the result 8

If I change the strtotal to strNumber1 / strNumber2 it gives the result 2

If I change the strtotal to strNumber1 - strNumber2 it gives the result 2

So what do I use to get it to add the numbers together .