Suppose you are constructing a finite-state machine for entering a security code into an automatic teller machine (ATM). The following rules are implemented.
A user enters a string of four digits, one digit at a time.
If the user enters the correct four digits of the password, the ATM displays a welcome screen
When the user enters an incorrect string of four digits, the ATM displays a screen that informs the user that an incorrect password was entered.
If a user enters the incorrect password three times, the account is locked
What is the input alphabet for this machine?
