On both sides of a 90-meter-long road, there used to be a street lamp every 9 meters (both ends), and now it is reinstalled, and one is installed every 6 meters. In order to save construction costs, the street lamps in some locations do not need to be reinstalled. How many street lights do not need to be reinstalled?
在一条长90米的公路两侧，原来每隔9米有一盏路灯(两端都有)，现重新安装，每隔6米装一盏，为节省施工成本有些位置的路灯不需要重新安装，不需要重新安装的路灯有多少盏？
<gadget id="calculator">90 / 18</gadget>
<output>5</output>

<gadget id="calculator">5 + 1</gadget>
<output>6</output>

<gadget id="calculator">6 * 2</gadget>
<output>12</output>

<result>12</result>