△○△○△○△○△As shown in the picture, there is one ○ in the middle of each △, and there are 20 △ in total like this, so how many ○ should be placed in the middle?
△○△○△○△○△如图，每个△中间有1个○，像这样一共摆20个△，那么中间一共要摆多少个○．
<gadget id="calculator">20 - 1</gadget>
<output>19</output>

<result>19</result>