There are 180 electric poles on both sides of a certain bridge from beginning to end, and the distance between two adjacent electric poles is 20 meters. How long is this bridge?
某大桥的两侧从头到尾共有180根电线杆，相邻两根电线杆间隔20米，这座大桥有多长？
<gadget id="calculator">180 / 2</gadget>
<output>90</output>

<gadget id="calculator">90 - 1</gadget>
<output>89</output>

<gadget id="calculator">20 * 89</gadget>
<output>1_780</output>

<result>1_780</result>