题目

 “Come on, Dad! It’s 7:20 a.m. now,” says Harry.  “I need to wash (洗) the car now,” Mr. Smith says. “You walk to school this morning, OK?”  “But the school is a little far,” says Jane.  “Walking is good. Now get your schoolbags and go!”   Then Jane and Harry walk to school. It takes 30 minutes. When they get to school, they are very tired.   In the evening, Jane and Harry have a big meal (饭) with their grandfather. They tell him they are not happy to walk to school. Grandpa says, “That’s good for your health. Kids (小孩) need to walk or run every day已知m=，n=，求m2﹣mn+n2的值．