If the sum of two prime numbers is 55, what is the difference between the two prime numbers?
两个质数的和为55，则这两个质数的差是多少．
<gadget id="calculator">55 - 2</gadget>
<output>53</output>

<gadget id="calculator">53 - 2</gadget>
<output>51</output>

<result>51</result>