=（998+1002）+（997+1003）+（996+1004）=6000
=1000-2+1000-3+1000-4+1000+2+1000+3+1000+4=6000
1000-2+1000-3+1000-4+1002+1003+1004=6000
998+1002+997+1003+996+1004≡6000
