Mr. Li's store sells two kinds of boxed lunches, each priced at an integer number of yuan, and type A is more expensive than type B. A guy with a very low education level records the amount of money (yuan) for each sale. Mr. Li saw that the first sum was 39, and the second sum was 61, so he said: You have made mistakes in these two sums, and it is impossible to calculate these two amounts. Seeing that the third stroke was 19, Mr. Li said that the number was correct. (It must be more than one box.) A friend of Mr. Li thought for a while and said that the unit price of the lunch box can be calculated from these three numbers, so how much is the box lunch of Type A?
李先生的店出售两种盒饭，每盒售价都是整数元，甲种比乙种贵．一个文化程度很低的伙计，把每笔出售的钱数(元)记录．李先生看第一笔是39，第二笔是61，就说：你这两笔都算错了，不可能算出这两个钱数．又看第三笔是19，李先生说这数算对了．(要算肯定不止一盒．)李先生的一位朋友想了一下，说从这三个数就可以推算盒饭的单价，那么甲种盒饭是多少元．
<result>11</result>