Question on transaction

I was asked a question in the interview..the question was like say 2 Users(say user A and user B) access a JSP page.This JSP page will display "count" and say currently the "count" value is 10.So both Users see "count" value as 10.Now this JSP has a "submit" button,and when the user clicks on that submit button,then their count should be increased by 1.Now the User A clicks on submit button,and the count increases by 1,so the count value becomes "11".But User B still sees 10,and now say User B hits submit button as well.Now instead of count increasing to "12",we should display an error message to the User B saying someone has already updated the "count" variable. How can this be achieved...

I thought either through timestamp or may be transaction..Anyone please share their views...