There are 780 students in grades one to six in a certain elementary school. Among the students who participated in the mathematics interest group study, exactly (8/17) were students in the sixth grade, and some (9/23) were students in the fifth grade. So, how many students in this school did not participate in the mathematics interest group?
某小学一至六年级共780名学生．在参加数学兴趣小组学习的学生中，恰有(8/17)是六年级的学生，有(9/23)是五年级的学生．那么，该校没有参加数学兴趣小组的学生有多少人？
<gadget id="calculator">780 - 391</gadget>
<output>389</output>

<result>389</result>