There are 60 people in a class, 42 of them can swim, 46 can ride a bicycle, 50 can skate, and 55 can play table tennis. How many people are sure to know at least four sports?
某班有60人，其中42人会游泳，46人会骑车，50人会溜冰，55人会打乒乓球．可以肯定至少有多少人四项运动都会？
<gadget id="calculator">60 - 42</gadget>
<output>18</output>

<gadget id="calculator">60 - 46</gadget>
<output>14</output>

<gadget id="calculator">60 - 50</gadget>
<output>10</output>

<gadget id="calculator">60 - 55</gadget>
<output>5</output>

<gadget id="calculator">18 + 14 + 10 + 5</gadget>
<output>47</output>

<gadget id="calculator">60 - 47</gadget>
<output>13</output>

<result>13</result>