There are 800 foreign language teachers in a certain city who understand English or Russian, among them 550 understand English, and 140 understand both English and Russian. How many teachers understand Russian?
某市800个外语教师懂英语或俄语，其中懂英语的550人，既懂英语又懂俄语的140人，那么懂俄语的教师为多少人．
<gadget id="calculator">550 - 140</gadget>
<output>410</output>

<gadget id="calculator">800 - 410</gadget>
<output>390</output>

<result>390</result>