If there are 1,000 foreign language teachers in a city who know English or Russian, 750 of them know English, and 200 know both English and Russian. How many teachers know Russian?
某市有1000个外语教师懂英语或俄语，其中懂英语的有750人，既懂英语又懂俄语的有200人，那么懂俄语的教师有多少人．
<gadget id="calculator">750 - 200</gadget>
<output>550</output>

<gadget id="calculator">1_000 - 550</gadget>
<output>450</output>

<result>450</result>