Assume that the number of pieces sold per month is X, and the total monthly income is Y yuan. For each piece sold, a reward is A yuan, and the monthly basic salary of sales staff is B yuan.
(1) Find the values of a and b;
(2) If the monthly income of Xiao Li, the shop assistant, is not less than 1800 yuan, how many clothes will Xiao Li sell this month at least?