a^2+b^2+c^2+ab+bc-ac =1/2[2a^2+2b^2+2c^2+2ab+2bc-2ac] =1/2[(a+b)^2+(a-c)^2+(b+c)^2] =1/2[(-2004+2003)^2+(-2003-(-2002))^2+(2003+ -2002)^2] =1/2[1+4+1] =3