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 = "# -*- coding: utf-8 -*-\n\nimport sys\n\ndef input(): return sys.stdin.readline().strip()\ndef list2d(a, b, c): return [[c] * b for i in range(a)]\ndef list3d(a, b, c, d): return [[[d] * c for j in range(b)] for i in range(a)]\ndef list4d(a, b, c, d, e): return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]\ndef ceil(x, y=1): return int(-(-x // y))\ndef INT(): return int(input())\ndef MAP(): return list(map(int, input().split()))\ndef LIST(N=None): return list(MAP()) if N is None else [INT() for i in range(N)]\ndef Yes(): print('Yes')\ndef No(): print('No')\ndef YES(): print('YES')\ndef NO(): print('NO')\nsys.setrecursionlimit(10 ** 9)\nINF = float('inf')\nMOD = 10 ** 9 + 7\n\nS = input()\nN = len(S)\n\n# nxt[i][c] := i文字目以降で最初に文字cが登場するindex(存在しないときはN)\nnxt = list2d(N+1, 26, N)\nfor i in range(N-1, -1, -1):\n    for c in range(26):\n        nxt[i][c] = nxt[i+1][c]\n    c = ord(S[i]) - 97\n    nxt[i][c] = i\n\n# dp[i] := i文字目以降について「部分文字列とならない」最短の文字列長\ndp = [INF] * (N+1)\n# rest[i][(c, j)] := 位置iから見て、最小達成に用いた文字cと、次に使う文字のindexj\nrest = [None] * (N+1)\ndp[N] = 1\n# 常に自分より後ろでの最小が必要になるので、後ろから埋めていく\nfor i in range(N-1, -1, -1):\n    # iから見て先頭として使う文字を26通り全部試す\n    # abcを昇順で回して、最小更新条件を等号なしにすることで、辞書順で小さい方を優先できる\n    for c in range(26):\n        # 次の文字がない時\n        if nxt[i][c] == N:\n            if dp[i] > 1:\n                dp[i] = 1\n                rest[i] = (c, N)\n        # 次の文字がある時\n        else:\n            # i以降に文字cが出現するindex+1の位置での最短文字列長 + 1\n            # 例えばaについてなら、a****となるので、****での最小とaの分で+1\n            if dp[i] > dp[nxt[i][c]+1] + 1:\n                dp[i] = dp[nxt[i][c]+1] + 1\n                rest[i] = (c, nxt[i][c]+1)\n\n# 復元\ni = 0\nans = []\nwhile i < N:\n    c, idx = rest[i]\n    ans.append(chr(c+97))\n    i = idx\nprint((''.join(ans)))\n"
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

```

Measured memory_bytes: 15877