All the boys in the class can play basketball or football, 15 of them can play basketball and 10 of them can play football. It is known that there are 19 boys in the class, so how many boys can only play football?
班里男生都会打篮球或者踢球，其中有15人会打篮球，10人会踢球．已知班里共有19名男生，那么只会踢足球的男生有多少名．
<gadget id="calculator">10 - 19</gadget>
<output>-9</output>

<gadget id="calculator">15 + (-9)</gadget>
<output>6</output>

<gadget id="calculator">10 - 6</gadget>
<output>4</output>

<result>4</result>