Measure the height of the bridge on the bridge. Fold the rope in half and hang it to the water surface, and there is still 4 meters left; fold the length of the rope three times and hang it to the water surface, and there is still 1 meter left. How long is the rope in meters?
在桥上测量桥高．把绳子对折后垂到水面，还余4米；把绳长3折后垂到水面，还余1米．绳长多少米．
<gadget id="calculator">2 * 4</gadget>
<output>8</output>

<gadget id="calculator">3 * 1</gadget>
<output>3</output>

<gadget id="calculator">8 - 3</gadget>
<output>5</output>

<gadget id="calculator">3 - 2</gadget>
<output>1</output>

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

<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<gadget id="calculator">4 * 2</gadget>
<output>8</output>

<gadget id="calculator">10 + 8</gadget>
<output>18</output>

<result>18</result>