How to do math addition in expression

Hi community,

This is such a silly question yet I can't figure it out. How do you do addition in an expression? 

I am trying to find the index of a specific parameter in a given string, then +x to the index number so that I can remove the parameter key name.

For example:

String: https://www.testsite.com?identifier=12345

I wrote an expression to:

substr(@{testsite.web.webPageDetails._tenantID.queryString}, (indexOf(@{testsite.web.webPageDetails._tenantID.queryString}, 'identifier=')) +11)

So I want the numbers after identifier= (hence the +11), but I am getting this issue: The expression is invalid : You cannot use the character '+' like this because it is a keyword of the language. Please, add quotes to fix your expression.

How do you do simple math in an expression in AJO?

Thanks.