Comparing 2 columns, looking for difference

[pglibra]

Hello,
I am trying to compare 2 columns of data and looking for the difference in column A in relation to data in column B.
For example,

A...........B
6610 164513
6610 164513
6520 164513

I want to identify the row that has 6520.
Hope that makes sense.
How do I write the formula?

Thanks
pg