[asterisk-users] parsing issue

[CDR]

I get an error when I execute this code
exten => rejected,n,Hangup($[-1*${Z}])

May  2 13:42:09] WARNING[23128]: ast_expr2.fl:468 ast_yyerror:
ast_yyerror():  syntax error: syntax error, unexpected $end, expecting
'-' or '!' or '(' or '<token>'; Input:
-1*

The variable "Z" has a  negative number, which is the code that I need
to use in the hangup.
Any idea how can I do this? There is no ABS() function in Asterisk. I
already filed a request for it but it turns up that it will cost me
money. How can I remove the sign from a number?
Philip