A bridge is 1800 meters long. A train passes the bridge at a speed of 500 meters per minute, and it takes 4 minutes from the front of the train to the bridge to the rear of the train leaving the bridge. How long is this train?
一座大桥长1800米．一列火车以每分钟500米的速度通过大桥，从车头开上桥到车尾离开桥共需要4分钟．这列火车长多少米．
<gadget id="calculator">4 * 500</gadget>
<output>2_000</output>

<gadget id="calculator">2_000 - 1_800</gadget>
<output>200</output>

<result>200</result>