The three natural numbers of 1989, 1999, and 2009 are respectively subtracted from the same positive integer a, and the differences obtained are all prime numbers, then a=how much.
1989、1999、2009三个自然数，分别减去同一个正整数a，得到的差均为质数，则a=多少．
<gadget id="calculator">1_989 - 3</gadget>
<output>1_986</output>

<result>1_986</result>