A subsequence of a string S is a string that can be obtained by deleting zero or more characters from S without changing the order of the remaining characters.
For example, arc, artistic and (an empty string) are all subsequences of artistic; abc and ci are not.
You are given a string A consisting of lowercase English letters.
Find the shortest string among the strings consisting of lowercase English letters that are not subsequences of A.
If there are more than one such string, find the lexicographically smallest one among them.

-----Constraints-----
 - 1 \leq |A| \leq 2 \times 10^5
 - A consists of lowercase English letters.

-----Input-----
Input is given from Standard Input in the following format:
A

-----Output-----
Print the lexicographically smallest string among the shortest strings consisting of lowercase English letters that are not subsequences of A.

-----Sample Input-----
atcoderregularcontest

-----Sample Output-----
b

The string atcoderregularcontest contains a as a subsequence, but not b.

```python

USER_FILENAME = "<user_code>"
import sys, json, builtins
import math, heapq, bisect, itertools, functools, collections, statistics, fractions, decimal, operator, array, random, re, string, os, pathlib
try:
    import resource as _res
except Exception:
    _res = None
from collections import Counter, defaultdict, deque, OrderedDict
from functools import lru_cache, reduce
from itertools import accumulate, permutations, combinations, product
from math import gcd, ceil, floor, sqrt
from decimal import Decimal
try:
    from typing import *
except Exception:
    pass
try:
    sys.setrecursionlimit(1_000_000)
except Exception:
    pass
try:
    import site as _site
    try:
        _sys_sites = _site.getsitepackages() if hasattr(_site, "getsitepackages") else []
        for _p in _sys_sites:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
    try:
        _usp = _site.getusersitepackages()
        if isinstance(_usp, str):
            _usp = [_usp]
        for _p in _usp:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
except Exception:
    pass

ns = {"__name__": "__main__"}
for _k, _v in list(globals().items()):
    if not _k.startswith("_"):
        ns[_k] = _v
_CODE = 'n2a=lambda x:chr(x+ord(\'a\'))\na2n=lambda x:ord(x)-ord(\'a\')\n\ndef main(a):\n  nc=26\n  n=len(a)\n  ary=[]\n  tmp=[0]*nc\n  mi=set(range(nc))\n  for i in range(n-1,-1,-1):\n    x=a2n(a[i])\n    mi.discard(x)\n    tmp[x]+=1\n    if len(mi)==0:\n      ary.append(tmp.copy())\n      tmp=[0]*nc\n      mi=set(range(nc))\n  if any(tmp):ary.append(tmp)\n  #for x in ary:print(x)\n  ans=[]\n  now=0\n  tmp=ary.pop()\n  if all(tmp):\n    ans.append(\'a\')\n    while a[now]!=\'a\':\n      tmp[a2n(a[now])]-=1\n      now+=1\n    tmp[a2n(a[now])]-=1\n    now+=1\n  else:\n    for i in range(nc):\n      if tmp[i]==0:\n        ans.append(n2a(i))\n        break\n    if not ary:return ans\n    ntmp=ary.pop()\n    for j in range(nc):tmp[j]+=ntmp[j]\n    while a2n(a[now])!=i:\n      tmp[a2n(a[now])]-=1\n      now+=1\n    tmp[a2n(a[now])]-=1\n    now+=1\n  while True:\n    #print(ary,tmp,ans)\n    for i in range(nc):\n      if tmp[i]==0:\n        ans.append(n2a(i))\n        break\n    if not ary:return ans\n    ntmp=ary.pop()\n    for j in range(nc):tmp[j]+=ntmp[j]\n    while a2n(a[now])!=i:\n      tmp[a2n(a[now])]-=1\n      now+=1\n    tmp[a2n(a[now])]-=1\n    now+=1\n\na=input()\nprint(*main(a),sep=\'\')\n"""\nbcdefghijklmnopqrstuvwxyza\n\naabbccaabca\naab / bccaa / bca\n\n"""\n\n'
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

```

Measured memory_bytes: 13243