There are 17 classes in the fourth, fifth, and sixth grades of a certain school, with 737 students. It is known that each class of the fourth grade has 41 students, each class of the fifth grade has 43 students, and each class of the sixth grade has 45 students. Then the school's How many classes are there at most in the fourth and fifth grades?
某校四、五、六年级共有17个班，737名学生，已知四年级每班都是41人，五年级每班都是43人，六年级每班都是45人，那么这个学校的四、五年级最多有多少个班．
<gadget id="calculator">17 - 4</gadget>
<output>13</output>

<result>13</result>