You'll do much better _____ you are more careful with your spelling.

Peter: Do you have a brother, John?

John: Yes, I do. His name is Mike.

Peter: Is he older than you?

John: No, he is younger than me. He is three years younger than me. He is 12 years old.

Peter: He is shorter than you, isn't he?

John: No, he is a little taller than me.

Peter: Are his eyes bigger than yours?

John: Yes, his eyes are bigger than mine.

Peter: Who loves sports more, you or your brother?

John: He does. He is good at playing football.

若实数x，y满足不等式组合$\left\{\begin{array}{c}x+3y-3\ge 0\\ 2x-y-3\le 0\\ x-y+1\ge 0\end{array}\right.$，则x+y的最大值为（　　）