The quotient and divisor of a division expression are both 20, what is the divisor to maximize the remainder. (The maximum fill in the horizontal line)
一个除法算式的商和除数都是20，要使余数最大，被除数是多少．(横线里最大填几)
<gadget id="calculator">20 * 20</gadget>
<output>400</output>

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

<gadget id="calculator">400 + 19</gadget>
<output>419</output>

<result>419</result>