return missing values

Hi,

I have two worksheets every with two column.
In the first sheet in Column A are 4digit codes, in Column B should be filled in required information(missing indexes) .
Sheet 2 is based on report for these 4 digit codes(column A) with existing indexes for these codes.
The values for the indexes are between 1-9999 and they were not used sequentially.
For example for code 1685 we can see in sheet2 that used indexes are 1,2,5,30,998 so in sheet 1 should be returned 3,4,6.

Can you please suggest vba code that return unused indexes?

Regards,
Guerolito