Message problem.

Hi,

I have a table which contains the student data.The student id is unique and should be in sequence like if we have 40 students and next student joins then his Id should be 41.Now in the dialog progarm user enters the student id.But I want when the user enters the student id other than 41 , he should get a message that , please enter the student id as 41(current no. +1).Please suggest how should I do it.