Fold a rectangular piece of paper in half, you can get a crease, and continue to fold in half. When folding in half, each crease is parallel to the last crease. After three consecutive folds in half, there are 7 creases, so fold it in half four times How much can be listed.
将一张长方形的纸片对折，可以得列一条折痕，继续对折，对折时，每次折痕与上次折痕保持平行，连续对折三次后，得列7条折痕，那么对折四次可以得列多少．
<gadget id="calculator">2 ** 4</gadget>
<output>16</output>

<gadget id="calculator">16 - 1</gadget>
<output>15</output>

<result>15</result>