There are 200 students in the first and second grades of Zhenhai Yale School, 250 students in the third and fourth grades, 220 students in the second and third grades, how many students in the first and fourth grades?
镇海雅乐学校一年级和二年级共200人，三年级和四年级共250人，二年级和三年级共220人，一年级和四年级共多少人．
<gadget id="calculator">250 - 220</gadget>
<output>30</output>

<gadget id="calculator">200 + 30</gadget>
<output>230</output>

<result>230</result>