I didn't notice it had worked out that way, but I only edited to add an example, not to change my stated understanding of his question (as stated in the unedited first line). But sorry for any confusion that may have added.OK Mike you have edited your previuos post after I posted.
I think the wording thing might just be that English isn't his first language.
Just thought of something else; It's even easier to spot gaps in a numeric sequence if you use something like this
=IF(A2<>A1+1,"<<Gap","")
1
2
3 <<Gap
5
6
7 <<Gap
9
10 <<Gap
probably means that your values are text not numbers
Ah. Yes - because of the string slicing exercise above.
Try:
=IF(Val(A2)<>(Val(A1)+1),"<<Gap","")