The sum of two non-zero natural numbers is 20, what is the minimum product of these two natural numbers
两个非零自然数的和是20，这两个自然数的积最小是多少
<gadget id="calculator">1 * 1</gadget>
<output>1</output>

<gadget id="calculator">20 - 1</gadget>
<output>19</output>

<result>19</result>