inserting part of IP ADDRESS with VAR

HI, here's a little scripting question: I'm trying to add an IP address and use a variable for 1 part of the IP. It seems I am not using the proper syntax. Here's my script so far:

THIS SCRIPT CREATE A VIRTUAL PRIVATE NETWORK

#DECLARING VARIABLES

:local ID;
:set ID "192.168.66.66/0";
:put $ID;

CREATING THE IP

/ip address add address 10.5.$ID.0 interface bridge-local netmask=255.255.255.0

So I'm expecting the address to be added to be 10.5.90.0 ( where 90 is the actual Variable)

thanks