There are 45 students in a certain class, 28 of them learn piano, 35 learn computer, 37 learn art, and 40 attend Olympiad School, so it is certain that at least how many students in this class learn all the above four contents up?
某班有学生45人，其中有28人学习钢琴，有35人学习电脑，有37人学习美术，有40人上奥校，那么可以肯定，这个班至少有多少个学生以上四项内容都学了？
<gadget id="calculator">45 - 28</gadget>
<output>17</output>

<gadget id="calculator">45 - 35</gadget>
<output>10</output>

<gadget id="calculator">45 - 37</gadget>
<output>8</output>

<gadget id="calculator">45 - 40</gadget>
<output>5</output>

<gadget id="calculator">17 + 10 + 8 + 5</gadget>
<output>40</output>

<result>5</result>