There are 17 classes with 737 students in grades four, five and six in a certain school. It is known that there are 45 students in each class of the fourth grade, 43 students in each class of the fifth grade, and 41 students in each class of the sixth grade. How many classes can there be at most in the fifth and sixth grades?
某校四五六三个年级共17个班，737名学生．已知四年级每班45人，五年级每班43人，六年级每班41人，五六年级最多共有多少个班．
<gadget id="calculator">17 - 4</gadget>
<output>13</output>

<result>13</result>