根据短文理解完成下列各题。

    Denzil wasn't a very clever donkey. He always did some silly things because he didn't think. "Look out," the other donkeys said. "Here comes Denzil—everybody runs away!"

    One day, Denzil carried two heavy bags of salt, one on each side of his back. When he was crossing a big river, he fell into the water. The salt began to melt. When he tried his best to stand up, all the salt was gone. "Hey, that's really great," thought Denzil. "I must remember that. Next time I can do it again when I'm crossing a river."

    The next time he carried two bags of cotton(棉花). "Watch this, everybody," said Denzil when he and his friends got to the middle of the river. He went into the water and was gone! Nobody told him that cotton could get heavier in water. The other donkeys pulled him out at once and luckily he was still alive.

设变量$x,y$满足约束条件$\{\begin{array}{c}y\ge x\\ x+2y\le 2\\ x\ge -2\end{array}$，则$z=x-3y$的最小值为（   ）