Crystal Reports remove brackets around a value

Message

     Hi 

I wish to remove a pair of brackets ( ) around a value in "tonumber({@get_value})" in the line in red:

if left(gvalue,1) <> '('  then
    sub_value_amt := sub_value_amt + tonumber({@get_value})
else
   sub_value_amt := sub_value_amt - tonumber({@get_value})

Note:
{@get_value} contains a cost something like '($5.00)' , it's a string, after removing the brackets, then it converts to number type for further subtraction.

Thanks in advance.

John