Liangliang and Congcong set off at the same time from two places 1,400 meters apart and walked towards each other. Liangliang walked 65 meters per minute, Congcong walked 75 meters per minute, and Congcong set off at the same time with a dog. He ran towards Liangliang at a speed of 210 meters. When he met Liangliang, he turned back and ran towards Congcong. After meeting Congcong, he turned back and ran towards Liangliang. The dog didn't stop until Liangliang and Congcong met. How many miles did the dog run in total?
亮亮、聪聪两人从相距1400米的两地同时出发相向而行，亮亮每分钟走65米，聪聪每分钟走75米，聪聪带了一只狗同时出发，狗以每分钟210米的速度向亮亮奔去，遇到亮亮后立即回头向聪聪奔去，遇到聪聪后又回头向亮亮奔去，直到亮亮、聪聪两人相遇时狗才停止．这只狗共奔跑了多少路程？
<gadget id="calculator">65 + 75</gadget>
<output>140</output>

<gadget id="calculator">1_400 / 140</gadget>
<output>10</output>

<gadget id="calculator">10 * 2_100</gadget>
<output>21_000</output>

<result>21_000</result>