# -*- coding: utf-8 -*-
pm = int(input('PM2.5的值为：'))
if 0 <= pm <= 35:
print('空气质量为优')
elif 35 < pm <= 75:
print('空气质量为良')
elif 75 < pm <= 115:
print('空气质量为轻度污染')
elif 115 < pm <= 150:
print('空气质量为中度污染')
elif 150 < pm <= 250:
print('空气质量为重度污染')
elif 250 < pm <= 500:
print('空气质量为严重污染')
