If the fifteenth number of a series of regularly arranged numbers 2, 1, 4, 3, 6, 5, 8, 7, ... is 16, what is the sum of its first fifteen numbers?
一串有规律排列的数2，1，4，3，6，5，8，7，…的第十五个数是16，则它的前十五个数的和是多少．
<gadget id="calculator">2 + 4 + 6 + 8 + 10 + 12 + 14 + 16</gadget>
<output>72</output>

<gadget id="calculator">1 + 3 + 5 + 7 + 9 + 11 + 13</gadget>
<output>49</output>

<gadget id="calculator">72 + 49</gadget>
<output>121</output>

<result>121</result>