In the physical education class, the teacher asked 42 students to stand in a line. Dongdong found that half of them were standing on his left; Archie found that he was the 12th from right to left. ?
体育课上老师让42名同学站成一行，冬冬发现有一半人站在他自己的左边；阿奇发现自己是从右往左数的第12个，冬冬和阿奇之间有多少人？
<gadget id="calculator">42 - 21 - 1</gadget>
<output>20</output>

<gadget id="calculator">12 - 1</gadget>
<output>11</output>

<gadget id="calculator">20 - 11 - 1</gadget>
<output>8</output>

<result>8</result>