class Solution:
def removeDuplicateLetters(self, s: str) -> str:
for i in sorted(set(s)):
suff = s[s.index(i):]
if set(suff) == set(s):
return i + self.removeDuplicateLetters(suff.replace(i, ''))
if s == '':
return s
a = Solution()
s = input()
s = a.removeDuplicateLetters(s)
print(s[0])