If the difference between two four-digit numbers is equal to 8921, it means that the two four-digit numbers form a pair. Question: How many such pairs are there?
如果两个四位数差等于8921，就是说这两个四位数组成一个数对．问：这样的数对共有多少个？
<gadget id="calculator">8_921 + 1_000</gadget>
<output>9_921</output>

<gadget id="calculator">9_999 - 9_921</gadget>
<output>78</output>

<gadget id="calculator">78 + 1</gadget>
<output>79</output>

<result>79</result>