The sum of three consecutive natural numbers is 39, what is the smallest of these three numbers?
三个连续自然数的和是39，这三个数中最小的是多少．
<gadget id="calculator">39 / 3</gadget>
<output>13</output>

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

<result>12</result>