A and B start from A and B respectively at the same time, and walk towards each other at a constant speed. A goes back immediately after arriving at B, and B also goes back immediately after arriving at A. It is known that they met for the first time when they were away from The midpoint of A and B is 2 kilometers away from B-side, and the second meeting is at 4 kilometers away from A. How many kilometers are the distance between A and B?
甲、乙二人同时分别从A、B两地出发，相向匀速而行，甲到达B地后立即往回走，乙到达A地后也立即往回走，已知他们第一次相遇在离A、B中点2千米处靠B-侧，第二次相遇在离A地4千米处，A、B两地相距多少千米．
<gadget id="calculator">2 * 3</gadget>
<output>6</output>

<gadget id="calculator">6 + 4</gadget>
<output>10</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(1/2) * 3</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">2 - (3/2)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">10 / (1/2)</gadget>
<output>20</output>

<result>20</result>