There is an east-west tunnel. To measure the length of the tunnel, A measures from east to west and draws a mark every 7 meters (including the starting point), and B measures from west to east and draws a mark every 9 meters (including the starting point). Among all these marks, the distance between the nearest two marks is 0.5 meters, and there are 31 minimum distances like this in the whole process. So how long is this tunnel at least?
有一东西向的隧道．为测量隧道的长度，甲自东向西测量，每隔7米画上一个记号(包括起点)，乙由西向东测量，每隔9米画上一个记号(包括起点)．在所有这些记号中，相距最近的两记号的距离为0.5米，全程像这样的最小距离共有31个，那么这条隧道至少有多少米长？
<gadget id="calculator">31 - 1</gadget>
<output>30</output>

<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<gadget id="calculator">7 * 9 * 15</gadget>
<output>945</output>

<gadget id="calculator">7 * 9</gadget>
<output>63</output>

<gadget id="calculator">63.5 - 63</gadget>
<output>0.5</output>

<gadget id="calculator">945 + 0.5</gadget>
<output>945.5</output>

<result>945.5</result>