There is a sequence: 1, 2, 4, 7, 11, 16, ... What is the 16th number in this sequence.
有一数列：1、2、4、7、11、16、…这列数列第16个数是多少．
<gadget id="calculator">1 + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15</gadget>
<output>121</output>

<result>121</result>