How to write TSQL insert stored procedure with max value of previous serial numbers?

I have Letters table:

+--------------+-------+
| SerialNumber | Letter|
+--------------+-------+
| 1            | A     |
| 2       开发者_StackOverflow     | B     |
| 3            | C     |
| 4            | D     |
+--------------+-------+

How to write TSQL insert stored procedure PA_Letters_INS which automatically adds max of previous serial number values and has letter insert parameter (without MSSQL autoincrement functionality on SerialNumber column).

(exec PA_Letters_INS 'E' adds {5, E} record)

@Letter being you Stored Procedure Parameter,

INSERT INTO Letters(SerialNumber, Letter)
     SELECT MAX(SerialNumber) + 1, @Letter 
     FROM Letters