Mingming used a rope to measure the depth of the well. He hung one end of the rope into the bottom of the well, and there was 15 meters outside the well. He folded the rope in half, and then hung one end into the bottom of the well. How deep is the well?
明明用绳测量井的深度，他把绳子一端垂入井底，井外多出了15米，他又把这根绳子对折后，将一端垂入井底，这时井口外的绳子还有2米，这口井有多深？
<gadget id="calculator">2 * 2</gadget>
<output>4</output>

<gadget id="calculator">15 - 4</gadget>
<output>11</output>

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

<gadget id="calculator">11 / 1</gadget>
<output>11</output>

<result>11</result>