There are 41 small flags inserted on the side of the straight road, and the interval between them is 3 meters. Now change to only insert 31 small flags (the small flags at both ends do not move), how many meters should the interval be changed?
笔直的公路一旁插有41面小旗，它们的间隔是3米．现在改为只插上31面小旗(两端小旗不动)，间隔应改为多少米？
<gadget id="calculator">41 - 1</gadget>
<output>40</output>

<gadget id="calculator">40 * 3</gadget>
<output>120</output>

<gadget id="calculator">31 - 1</gadget>
<output>30</output>

<gadget id="calculator">120 / 30</gadget>
<output>4</output>

<result>4</result>