将百分制分数mark转换为五级制（优、良、中、及格、不及格）的评定等级grade

mark = int(input('请输入x='))
if mark >=90:
    print('优')
elif 80<=mark <=90:
    print('良')
elif 70<=mark <=80:
    print('中')
elif 60<=mark <=70:
    print('及格')
elif mark <=60:
    print('不及格')