求教各位大神，请问有什么错误。系统提示索引超出范围，大概就是标红的地方，求教

[ilove_python]

import urllib.request as req
from lxml import etree
i =2
while i<=6:
html =req.urlopen("http://site.gzcc.cn/html/szdw/"+str(i)+".html")
content =html.read()
selectors =etree.HTML(content)
j=1
while j<=7:
list1 =selectors.xpath("/html/body/div[4]/div/div[1]/div[2]/ul/a["+str(j)+"]/li/div/div[2]/p[1]/img")
list2 =selectors.xpath("/html/body/div[4]/div/div[1]/div[2]/ul/a["+str(j)+"]/li/div/div[2]/p[2]/img")
f =open("test.txt","a")
f.write(list1[0]+"--------"+list2[0]+"\n")
f.close()
j+=1
i+=1