The seven numbers 8, 9, 10, 11, 12, 13, and 14 are written on the blackboard. Erase two numbers arbitrarily each time, and then write the sum of these two numbers minus 1. For example, to erase 9 and 13, write 21. After several times, there will be only one number left on the blackboard, what is this number.
黑板上写着8，9，10，11，12，13，14七个数，每次任意擦去两个数，再写上这两个数的和减1．例如，擦掉9和13，要写上21．经过几次后，黑板上就会只剩下一个数，这个数是多少．
<gadget id="calculator">9 - 1</gadget>
<output>8</output>

<gadget id="calculator">8 + 8</gadget>
<output>16</output>

<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<gadget id="calculator">16 + 9</gadget>
<output>25</output>

<gadget id="calculator">11 - 1</gadget>
<output>10</output>

<gadget id="calculator">25 + 10</gadget>
<output>35</output>

<gadget id="calculator">12 - 1</gadget>
<output>11</output>

<gadget id="calculator">35 + 11</gadget>
<output>46</output>

<gadget id="calculator">13 - 1</gadget>
<output>12</output>

<gadget id="calculator">46 + 12</gadget>
<output>58</output>

<gadget id="calculator">14 - 1</gadget>
<output>13</output>

<gadget id="calculator">58 + 13</gadget>
<output>71</output>

<result>71</result>