There are 100 foreign language teachers in a certain area, 85 teachers who understand English, 10 who understand both English and Russian, how many teachers understand Russian?
某地区有外语老师100名，懂英语的老师85名，既懂英语又懂俄语的有10人，懂俄语的老师多少人．
<gadget id="calculator">85 - 10</gadget>
<output>75</output>

<gadget id="calculator">100 - 75</gadget>
<output>25</output>

<result>25</result>