Put eight numbers in a row from left to right, starting from the third number, each number is exactly equal to the sum of the two numbers before it, if the seventh and eighth numbers are 81, 131 respectively, then What is the first number.
将八个数从左到右列成一行，从第三个数开始，每个数都恰好等于它前面两个数之和，如果第7个数和第8个数分别是81，131，那么第一个数是多少．
<gadget id="calculator">131 - 81</gadget>
<output>50</output>

<gadget id="calculator">81 - 50</gadget>
<output>31</output>

<gadget id="calculator">50 - 31</gadget>
<output>19</output>

<gadget id="calculator">31 - 19</gadget>
<output>12</output>

<gadget id="calculator">19 - 12</gadget>
<output>7</output>

<gadget id="calculator">12 - 7</gadget>
<output>5</output>

<result>5</result>