File Template integer parameter/variable

Hi,

I'd like to pass an integer variable to my file template, but it is interpreted as string:

#set( $inc = $a + 1 )

a  = $a

a + 1 = $inc

Create template with a = 2 and you get:

a = 1

a + 1 = 21

I expect (like to get):

a = 1

a + 1 = 3

How can I achieve this?

Thanks,

Michael