2008-2+2004-6+2000-10+~+12-1998+8-2002+4-2006
=2008+2004+2000+...+8+4-(2+6+...+2006)
=(2008-2006)+(2004-2002)+...+(4-2)
=2+2+...+2 {(2008-4)/4+1=502个2}
=2*502
=1004