Mary works in a top secret research laboratory. In order to gain access to the building, she must use a secret four-digit code. All codes from 0000 to 9999 are possible. As a security feature Mary is required to get exactly one digit wrong each time, and not to enter the same four-digit number as she did last time. The security guard can see the numbers that she enters into the keypad, What is the maximum number of different four-digit numbers Mary can enter without the security guard being able to be certain of the secret code? 44LILI