a=['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z','1','2','3','4','5','6','7','8','9']
for i in range(len(a)):
 print a[i]

aa
ab
ac
ad
...
ba
bb
bc
...
za
zb

aaa
aab
...
aba
abb
...
zaa
zab