(1+3+5+ ``` ＋1999+2000+2001)-(2+4+6+```+1998+2002）
=(1-2)+(3-4)+(5-6)+```+(2001-2002)
=-1*2002/2
=-1001
99`````````9 Ⅹ 99`````````9+1999```````9
（2000个9） （2000个9） （2000个9）
=(10`````````0-1) Ⅹ 99`````````9+1999```````9
（2000个0） （2000个9） （2000个9）
=99`````````900`````````0-
(2000个9) (2000个0)
99`````````9+199`````````9
(2000个9) (2000个9)
=99`````````900`````````0+100`````````0
(2000个9) (2000个0) (2000个0)
=10000``````````````````00
(4000个0)