Normal assets are represented by C, and Mn means overdue by n periods: M 1 overdue by one period, M2 overdue by two periods, M3 overdue by three periods, M4 overdue by four periods, M5 overdue by five periods, M6 overdue by six periods, Mn+ representing overdue by more than n periods, and M7+ representing overdue periods "=M7 3. The loan balance ENR reaches the borrower's outstanding principal at a certain point, that is, all the remaining principal is regarded as the loan balance.