Among the students from the navy department, how many of them have payment due?
from the navy department refers to organ = 'navy'; have payment due refers to bool = 'pos';
SELECT COUNT(T1.name) FROM enlist AS T1 INNER JOIN no_payment_due AS T2 ON T1.`name` = T2.`name` WHERE T1.organ = 'navy' AND T2.bool = 'pos'