Draw 20 circles on the plane, and ask how many parts the plane can be divided into by these 20 circles.
在平面上画20个圆，问这20个圆最多可能将平面分为多少个部分．
<gadget id="calculator">20 * 19</gadget>
<output>380</output>

<gadget id="calculator">2 + 380</gadget>
<output>382</output>

<result>382</result>