Kostya likes Codeforces contests very much. However, he is very disappointed that his solutions are frequently hacked. That's why he decided to obfuscate (intentionally make less readable) his code before upcoming contest.

To obfuscate the code, Kostya first looks at the first variable name used in his program and replaces all its occurrences with a single symbol *a*, then he looks at the second variable name that has not been replaced yet, and replaces all its occurrences with *b*, and so on. Kostya is well-mannered, so he doesn't use any one-letter names before obfuscation. Moreover, there are at most 26 unique identifiers in his programs.

You are given a list of identifiers of some program with removed spaces and line breaks. Check if this program can be a result of Kostya's obfuscation.
In the only line of input there is a string *S* of lowercase English letters (1 ≤ |*S*| ≤ 500) — the identifiers of a program with removed whitespace characters.
In the first sample case, one possible list of identifiers would be "number string number character number string number". Here how Kostya would obfuscate the program:
-  replace all occurences of number with a, the result would be "a string a character a string a",-  replace all occurences of string with b, the result would be "a b a character a b a",-  replace all occurences of character with c, the result would be "a b a c a b a",-  all identifiers have been replaced, thus the obfuscation is finished.
abacaba
YES
jinotega
NO
aaaaaaaaaaa
YES
aba
YES
bab
NO
a
YES
abcdefghijklmnopqrstuvwxyz
YES
fihyxmbnzq
NO
aamlaswqzotaanasdhcvjoaiwdhctezzawagkdgfffeqkyrvbcrfqgkdsvximsnvmkmjyofswmtjdoxgwamsaatngenqvsvrvwlbzuoeaolfcnmdacrmdleafbsmerwmxzyylfhemnkoayuhtpbikm
NO
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
YES
darbbbcwynbbbbaacbkvbakavabbbabzajlbajryaabbbccxraakgniagbtsswcfbkubdmcasccepybkaefcfsbzdddxgcjadybcfjtmqbspflqrdghgfwnccfveogdmifkociqscahdejctacwzbkhihajfilrgcjiofwfklifobozikcmvcfeqlidrgsgdfxffaaebzjxngsjxiclyolhjokqpdbfffooticxsezpgqkhhzmbmqgskkqvefzyijrwhpftcmbedmaflapmeljaudllojfpgfkpvgylaglrhrslxlprbhgknrctilngqccbddvpamhifsbmyowohczizjcbleehfrecjbqtxertnpfmalejmbxkhkkbyopuwlhkxuqellsybgcndvniyyxfoufalstdsdfjoxlnmigkqwmgojsppaannfstxytelluvvkdcezlqfsperwyjsdsmkvgjdbksswamhmoukcawiigkggztr
NO
bbbbbb
NO
aabbbd
NO
abdefghijklmnopqrstuvwxyz
NO
abcdeghijklmnopqrstuvwxyz
NO
abcdefghijklmnopqrsuvwxyz
NO
abcdefghijklmnopqrstuvwxy
YES
abcdefghijklmnopqrsutvwxyz
NO
acdef
NO
z
NO
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaababaaababaabababccbabdbcbadccacdbdedabbeecbcabbdcaecdabbedddafeffaccgeacefbcahabfiiegecdbebabhhbdgfeghhbfahgagefbgghdbhadeicbdfgdchhefhigfcgdhcihecacfhadfgfejccibcjkfhbigbealjjkfldiecfdcafbamgfkbjlbifldghmiifkkglaflmjfmkfdjlbliijkgfdelklfnadbifgbmklfbqkhirhcadoadhmjrghlmelmjfpakqkdfcgqdkaeqpbcdoeqglqrarkipncckpfmajrqsfffldegbmahsfcqdfdqtrgrouqajgsojmmukptgerpanpcbejmergqtavwsvtveufdseuemwrhfmjqinxjodddnpcgqullrhmogflsxgsbapoghortiwcovejtinncozk
NO
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
YES
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbabbbabbaaabbaaaaabaabbaa
YES
aababbabbaabbbbbaabababaabbbaaaaabbabbabbaabbbbabaabbaaababbaaacbbabbbbbbcbcababbccaaacbaccaccaababbccaacccaabaaccaaabacacbaabacbaacbaaabcbbbcbbaacaabcbcbccbacabbcbabcaccaaaaaabcbacabcbabbbbbabccbbcacbaaabbccbbaaaaaaaaaaaadbbbabdacabdaddddbaabbddbdabbdacbacbacaaaabbacadbcddddadaddabbdccaddbaaacbceebbceadbeaadecddbbbcaaecbdeaebaddbbdebbcbaabcacbdcdc
YES
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbaabaabaababbbabbacacbbbacbbaaaabbccacbaabaaccbbbbbcbbbacabbccaaabbaaacabcbacbcabbbbecbecadcbacbaadeeadabeacdebccdbbcaecdbeeebbebcaaaeacdcbdeccdbbdcdebdcbdacebcecbacddeeaebcedffedfggbeedceacaecagdfedfabcfchffceachgcbicbcffeeebgcgiefcafhibhceiedgbfebbccegbehhibhhfedbaeedbghggffehggaeaidifhdhaggdjcfjhiaieaichjacedchejg
NO
b
NO
ac
NO
cde
NO
abd
NO
zx
NO
bcd
NO
aaac
NO
aacb
NO
acd
NO
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaz
NO
abcdefghijklmnopqrstuvwxyzz
YES
bc
NO
aaaaaaaaad
NO
abb
YES
abcb
YES
aac
NO
abcbcb
YES
bb
NO
abbb
YES
bbb
NO
x
NO
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaazz
NO
acbccccccccccc
NO
za
NO
ade
NO
bbbbbbbbbb
NO
bac
NO
bcddcb
NO
aaacb
NO
aaaaac
NO
aaaaaaaaaaad
NO
c
NO
abcccccccc
YES
aaaaaaac
NO
from string import ascii_lowercase

def can_decrypt(string):
    cur_idx = 0
    seen = set()

    for c in encoded:
        if c in seen:
            continue
        else:
            if c != ascii_lowercase[cur_idx]:
                return False
            else:
                seen.add(ascii_lowercase[cur_idx])
                cur_idx += 1
    return True

encoded = input()
if can_decrypt(encoded):
    print('YES')
else:
    print('NO')