There is one more male student in a group of 4, and one more male student in a group of 5. How many male students are there at least in this class?
某班的男同学，每4人一组多1人，每5人一组也多1人，这个班至少有男生多少人？
<gadget id="calculator">4 * 5</gadget>
<output>20</output>

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

<result>21</result>