There are 50 people in the whole class, 23 people can't ride a bicycle, 35 people can't skate, 4 people can do both, so how many people can't do both?
全班共有50人，不会骑车的有23人，不会滑冰的有35人，两样都会的有4人，那么两样都不会的有多少人？
<gadget id="calculator">50 - 23 - 4</gadget>
<output>23</output>

<gadget id="calculator">50 - 35 - 4</gadget>
<output>11</output>

<gadget id="calculator">50 - 23 - 11 - 4</gadget>
<output>12</output>

<result>12</result>