The distance between the two places is 400 kilometers. There are two trains departing oppositely from these two places at the same time, and train A travels 45 kilometers per hour. Car B travels 35 kilometers per hour. A swallow starts at the same time as car A at a speed of 50 kilometers per hour, and flies towards car B. When it encounters car B, it turns back and flies to car A. The car flew away, and so on. How many kilometers did the swallow fly when the two trains met?
两地距离400千米．有两列火车从这两地同时相对开出，甲车每小时行45千米．乙车每小时行35千米．一只燕子以每小时50千米的速度与甲车同时同向出发，向乙车飞去，它遇到乙车后，又折回向甲车飞去，再遇到甲车后，又往乙车飞去，如此继续下去．求两列火车相遇时，这只燕子一共飞行多少千米？
<gadget id="calculator">45 + 35</gadget>
<output>80</output>

<gadget id="calculator">400 / 80</gadget>
<output>5</output>

<gadget id="calculator">5 * 50</gadget>
<output>250</output>

<result>250</result>