There is a box of apples, 5 numbers of 5, 4 numbers of 4, and 1 left in the end. How many apples are there at least in this box?
有一箱苹果，5个5个的数，4个4个的数，最后都余1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>