显然有：a = b-1 ,c = b+1 ,
可得：a+c = 2b ,ac = b^2-1 ；
所以,a+b+c-ab-ac-bc = (a+c)+b-b(a+c)-ac = 1+3b-3b^2 = 1+3b(1-b) = -12126329 .