  We study codes that can detect the exact number of deletions and insertions
in concatenated binary strings. We construct optimal codes for the case of
detecting up to $\del$ deletions. We prove the optimality of these codes by
deriving a converse result which shows that the redundancy of our codes is
asymptotically optimal in $\del$ among all families of deletion detecting
codes, and particularly optimal among all block-by-block decodable codes. For
the case of insertions, we construct codes that can detect up to $2$ insertions
in each concatenated binary string.
