Dr. Long put 48 crystal balls numbered 1-48 in a box. Oou randomly draws crystal balls from the box. How many crystal balls must he draw at least to ensure that there must be 2 crystal balls with a difference of 4.
龙博士在一个盒子里放有编号为1～48的48个水晶球．欧欧从盒子里任意抽取水晶球，他至少要抽出多少个水晶球，才能保证取出的水晶球里一定有2个编号之差为4．
<gadget id="calculator">24 + 1</gadget>
<output>25</output>

<result>25</result>