StoredProcedure:How to subtact previous value from recent value that value displayed in another column

Question

How to subtact previous value from recent value that value displayed in another column
in my database i have 4 columns
actually my task is differnt so iam taking some example
i have to insert data like
EmpID,EMpname,Sal,Expectedsal

when i insert data first like sal 300
the table should be
Empid,name,sal,expsal
1 k 300 0
again wheni insert data like sal 500
table should be
EMpid,name,sal,expsal
1 k 500 0
2 k 300 200
if their more than 500 records how to write storedprocedure
plz help me