If 100 foreign language teachers in a district know English or Russian, 75 of them know English, and 20 know both English and Russian. How many teachers know Russian?
某区100个外语教师懂英语或俄语，其中懂英语的75人，既懂英语又懂俄语的20人，那么懂俄语的教师为多少人．
<gadget id="calculator">75 - 20</gadget>
<output>55</output>

<gadget id="calculator">100 - 55</gadget>
<output>45</output>

<result>45</result>