If a is a natural number and the value of (20/(a+1)) is an integer, what is the sum of all the values of a that satisfy the condition?
a为自然数，(20/(a+1))的值为整数，则满足条件的所有a的值的和为多少．
<gadget id="calculator">0 + 1 + 3 + 4 + 9 + 19</gadget>
<output>36</output>

<result>36</result>