For a four-digit number, a decimal point is placed at a certain place in it, and a decimal is obtained. The sum of the four-digit number and the resulting decimal is 2000.81. What is the number of these four-digit numbers?
一个四位数，在它的某一位点上小数点，得到一个小数，这个四位数加上这个所得的小数，和是2000.81，这四位数是多少．
<gadget id="calculator">100 + 1</gadget>
<output>101</output>

<gadget id="calculator">2_000.81 / 101</gadget>
<output>19.81</output>

<gadget id="calculator">19.81 * 100</gadget>
<output>1_981</output>

<result>1_981</result>