The students do exercises. Xiao Lin stands on the 7th row from the left, the 13th row from the right, the 8th from the front, and the 14th from the back. Each row has the same number of people. How many students do exercises?
同学们做操．小林站在左起第7行，右起第13行，从前边数是第8个，从后边数是第14个．每行的人数同样多．做操的同学一共有多少个．
<gadget id="calculator">13 - 1</gadget>
<output>12</output>

<gadget id="calculator">7 + 12</gadget>
<output>19</output>

<gadget id="calculator">14 - 1</gadget>
<output>13</output>

<gadget id="calculator">8 + 13</gadget>
<output>21</output>

<gadget id="calculator">19 * 21</gadget>
<output>399</output>

<result>399</result>