minimize (a-\tfrac{a+b+(n-b)+(m-a)}{4})^2+(b-\tfrac{a+b+(n-b)+(m-a)}{4})^2+((n-b)-\tfrac{a+b+(n-b)+(m-a)}{4})^2+((m-a)-\tfrac{a+b+(n-b)+(m-a)}{4})^2 with respect to a and b