Optimization & Lagrange Multipliers · Medium problem

Let $T$ be the closed triangular region with vertices $(0,0)$, $(4,0)$ and $(0,4)$. Let $M$ and $m$ be the absolute maximum and absolute minimum of $f(x,y) = x^2 + y^2 - 2x - 2y$ on $T$. Find $M - m$.