Variance from a joint MGF

Let $(X,Y)$ be a random vector with joint moment generating function $$M(t_{1},t_{2})=\frac{1}{\left(1-(t_{1}+t_{2})\right)\left(1-t_{2}\right)},\quad-\infty<t_{1}<\infty, -\infty<t_{2}<\min\{1,1-t_{1}\}.$$ Let $Z=X+Y$. Then, $\operatorname{Var}(Z)$ is equal to