I need sequence diagram for the 4 objects( UI, Calendar, Appointment list, Reminder )

Given The following scenario: "The scenario begins when the user chooses to add a new appointment in the UI. The UI notices which part of the calendar is active and pops up an Add Appointment window for that date and time. The user enters information about the appointment's name, location, start and end times. The UI will prevent the user from entering an appointment that has invalid information, such as an empty name or negative duration. The calendar records the new appointment in the user's list of appointments. Any reminder selected by the user is added to the list of reminders. If the user already has an appointment at that time, the user is shown a message and asked to choose an available time or replace the appointment. If the user enters an appointment with the same name and duration as an existing meeting, the calendar asks the user whether he/she intended to join that meeting instead. If so, the user is added to that meeting's list of participants. a- Identify the main objects/classes in the system (at least 4). b- Draw sequence diagram for "add calendar appointment