A series of numbers composed of several 0s and 1s (such as 001, 1101, etc.) is called "dragon". The total number of 0s and 1s in "Dragon" is the length of "Dragon". A "Dragon" with exactly two 0s and 1 at the beginning and end is called a "True Dragon". Then a "True Dragon" with a length of 8 has how many.
将若干个0和1组成的一列数(如001，1101等)称为"龙"．"龙"中0和1的总个数为"龙"的长度，其中恰有两个0且首尾都是1的"龙"称为"真龙"，那么长度为8的"真龙"有多少条．
<gadget id="calculator">5 + 10</gadget>
<output>15</output>

<result>15</result>