请问二级 BOOK 4 <Alternative asset valuation and fixed income>里第18页，figure 5中第二行“less outstanding mortgage balance (from amortizaiotn)" 的这个数409，799是怎么算出来了，我看备注了，说他等于face value of loan -loan service payments reduced by interest paid,但是怎么算都不是这个数，请教高人指点！万分感谢！