Two couriers A and B set off from the east and west towns at a fixed speed at the same time. The second meeting at a place 500 meters away from the town, how many meters are the distance between the east and west towns?
甲、乙两名快递员分别以固定的速度同时从东、西两镇相对开出，并在两地间做往返运动，已知在距离东镇1800米的地方第一次相遇，在距离西镇500米的地方第二次相遇，那么东西两镇相距多少米？
<gadget id="calculator">1_800 * 2</gadget>
<output>3_600</output>

<gadget id="calculator">1_800 + 3_600</gadget>
<output>5_400</output>

<gadget id="calculator">5_400 - 500</gadget>
<output>4_900</output>

<result>4_900</result>