A bank has one million clients, one of whom is Inspector Gadget. Each client has a unique PIN number consisting of six digits. Dr. Claw has a list of all the clients. He is able to break into the account of any client, choose any $n$ digits of the PIN number and copy them. The n digits he copies from different clients need not be in the same $n$ positions. He can break into the account of each client, but only once. What is the smallest value of $n$ which allows Dr.Claw to determine the complete PIN number of Inspector Gadget?