There is a box of apples, if you take 4 pieces by 4, the result is 1 more; if you take 5 pieces by 5, the result is also 1 more. How many apples are there in this box?
有一箱苹果，如果4个4个地拿，结果多1个；如果5个5个地拿，结果也多一个．这箱苹果至少有多少个？
<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<gadget id="calculator">20 + 1</gadget>
<output>21</output>

<result>21</result>