There are 50 students in a certain class, 35 of them can swim, 38 can ride a bicycle, 40 can skate, and 46 can play table tennis. How many students in this class can be sure that they can do all four events?
某班有学生50人，其中35人会游泳，38人会骑车，40人会溜冰，46人会乒乓球，可以肯定这个班至少有几个学生四项都会？
<gadget id="calculator">50 - 35</gadget>
<output>15</output>

<gadget id="calculator">50 - 38</gadget>
<output>12</output>

<gadget id="calculator">50 - 40</gadget>
<output>10</output>

<gadget id="calculator">50 - 46</gadget>
<output>4</output>

<gadget id="calculator">15 + 12 + 10 + 4</gadget>
<output>41</output>

<gadget id="calculator">50 - 41</gadget>
<output>9</output>

<result>9</result>