The relation schema, \( \text{Person(pid, city) \), describes the city of residence for every person uniquely identified by \( \text{pid} \). The following relational algebra operators are available: \texttt{selection}, \texttt{projection}, \texttt{cross product}, and \texttt{rename}.}
To find the list of cities where at least 3 persons reside, using the above operators, the minimum number of \texttt{cross product operations that must be used is ...........\_.}