#Person1#: May I have my bill, please? 
#Person2#: That's $20 in all. 
#Person1#: Well, I am afraid you have made a mistake. Would you check it again, please? 
#Person2#: Let me see. I am terribty sorry, sir. That should be $18. 
#Person1#: It doesn't matter. 
#Person1# points out a mistake on #Person1#'s bill.