# your code goes here
s=input()
def LNDS(s):
    start = 0
    cur_len = 1
    max_len = 1
    for i in range(1,len(s)):
        if ord(s[i]) in (ord(s[i-1]), ord(s[i-1])+1):
            cur_len += 1
        else:
            if cur_len > max_len:
                max_len = cur_len
                start = i - cur_len
            cur_len = 1
    if cur_len > max_len:
        max_len = cur_len
        start = i - cur_len
    return s[start:start+max_len]
print(LNDS(s))

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

aGVsYWJjd29yZGVmZ2h3b3JkZWZnaGlq

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