There are 45 people in the second class of the fourth grade, 21 people can play chess, 17 people can play Go, and 10 people can't play both kinds of chess. How many people can play both kinds of chess?
四年级二班有45人，会下象棋的有21人，会下围棋的有17人，两种棋都不会的有10人．两种棋都会的有多少人？
<gadget id="calculator">10 - 45</gadget>
<output>-35</output>

<gadget id="calculator">21 + 17 + (-35)</gadget>
<output>3</output>

<result>3</result>