compare 2 values in textboxes

Hi,
I have about 20 rows in my page and each row has 2 textboxes -txtStartingNumber txtEndingNumber.
The textboxes are integers. My boss wants me to do custom validation on the textboxes to check that txtStartingNumber is always smaller than txtEndingNumber.
He said something about using Sender. I am not sure how to do it. Any help please?

Thanks