-2a^2-2b^-2c^+2ab+2ac+2bc
=-(2a^2+2b^+2c^-2ab-2ac-2bc)
=-[(a^2+b^2-2ab)+(b^2+c^2-2bc)+(a^2+c^2-2ac)]
=-(a-b)^2-(b-c)^2-(c-a)^2