In addition, citizens can be expected to provide the personal identification number (CNP) in many circumstances; purposes range from simple unique identification and internal book-keeping (for example when drawing up the papers for the warranty of purchased goods) to being asked for identification by the police. The CNP is 13 characters long, with the format S-YY-MM-DD-RR-XXX-Y. Where S is the sex, YY is year of birth, MM is month of birth, DD is day of birth, RR is a regional id, XXX is a unique random number and Y is a control digit.