eripatetic Shipping Lines, Inc., is a shipping company that owns n ships and provides service to n ports. Each of its ships has a schedule that says, for each day of the month, which of the ports it$$s currently visiting, or whether it$$s out at sea. $($You can assume the $$month$$ here has m days, for some m $>$ n$.)$ Each ship visits each port for exactly one day during the month. For safety reasons, PSL Inc. has the following strict requirement: