# 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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