6. Xiao Ming plans to memorize 40 English words from Monday to Sunday over 7 days. He requires himself to memorize a few words each day, with a different number of words memorized each day, and the most words are to be memorized on Sunday. How many words can Xiao Ming memorize at most on Sunday? $\qquad$
【Analysis】 $40-1-2-3-4-5-6=19$