Write the integers 1, 2, 3, ..., 100 on the blackboard. At least how many numbers must be erased so that the final digit of the product of all the numbers left on the blackboard is 2.
将整数1、2、3、…、100写在黑板上．至少要擦掉多少个数才能使得留在黑板上的全部数的乘积末位数是2．
<gadget id="calculator">10 + 10 + 1</gadget>
<output>21</output>

<result>21</result>