Fold a piece of square paper in half along a certain direction, then fold in half, ..., how many creases are there after folding in half 4 times.
将一张正方形纸沿着某个方向对折，再对折，…，对折4次后有多少条折痕．
<gadget id="calculator">1 + 2 + 4 + 8</gadget>
<output>15</output>

<result>15</result>