Find Missing Page numbers in oracle Apex Application

This code will help You to find the missing page number. Suppose we deleted some page between 1 to 900. Then we can now what are the page number available. So while copying any page , we can utilize that number.
This code will return the missing page no between 1 to 900(it will be change as per Your Application page number for example-1 to 10,10 to 100 like wise.)