There is a series of formulas: 100-1, 99-2, 98-3, 97-4... What is the value of the 40th formula?
有一串式子：100-1、99-2、98-3、97-4…则第40个式子的值是多少．
<gadget id="calculator">40 - 1</gadget>
<output>39</output>

<gadget id="calculator">100 - 39</gadget>
<output>61</output>

<gadget id="calculator">61 - 40</gadget>
<output>21</output>

<result>21</result>