用Python写一个程序

用Python编程，根据下表标准，输入PM2.5值，输出对应等级，输入-1结束

PM=0
while(PM !=-1):
    PM = eval(input("请输入PM2.5数值："))
    if 0<=PM< 50:
        print("1级")
    if 51<=PM<100:
        print("2级")
    if 101<=PM<150:
        print("3级")
    if 151<=PM<200:
        print("4级")
    if 201<=PM<300:
        print("5级")
    if PM>300:
        print("6级")