The sum of two prime numbers is 25, what is the difference between these two prime numbers.
两个质数的和是25，这两个质数的差是多少．
<gadget id="calculator">25 - 2</gadget>
<output>23</output>

<gadget id="calculator">23 - 2</gadget>
<output>21</output>

<result>21</result>