Two ferry boats A and B travel back and forth between A and B. They set off from both sides of the river at the same time, meet for the first time at a distance of 700 meters from A, and then continue to move forward at the same speed until they reach the opposite bank. They returned immediately, and they met for the second time at 400 meters from Bank B. How wide is the river?
A、B两只渡船在甲、乙两地间往返行驶，它们分别从河的两岸同时出发，在离甲岸700米处第一次相遇，然后继续以原速度前进，一直到达对岸后两船又立即返回，结果在乙岸400米处第二次相遇．问这条河有多宽？
<gadget id="calculator">700 * 2</gadget>
<output>1_400</output>

<gadget id="calculator">700 + 1_400</gadget>
<output>2_100</output>

<gadget id="calculator">2_100 - 400</gadget>
<output>1_700</output>

<result>1_700</result>