Problem:
Score : 600 points

Problem Statement

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 1

atcoderregularcontest

Sample Output 1

b

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

Sample Input 2

abcdefghijklmnopqrstuvwxyz

Sample Output 2

aa

Sample Input 3

frqnvhydscshfcgdemurlfrutcpzhopfotpifgepnqjxupnskapziurswqazdwnwbgdhyktfyhqqxpoidfhjdakoxraiedxskywuepzfniuyskxiyjpjlxuqnfgmnjcvtlpnclfkpervxmdbvrbrdn

Sample Output 3

aca

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>

using namespace std;

#define SZ(x) (int)(x.size())
#define REP(i, n) for(int i=0;i<(n);++i)
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define RREP(i, n) for(int i=(int)(n);i>=0;--i)
#define RFOR(i, a, b) for(int i=(int)(a);i>=(int)(b);--i)
#define ALL(a) (a).begin(),(a).end()
#define DUMP(x) cerr<<#x<<" = "<<(x)<<endl
#define DEBUG(x) cerr<<#x<<" = "<<(x)<<" (L"<<__LINE__<<")"<< endl;

using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using P = pair<int, int>;

const double eps = 1e-8;
const ll MOD = 1000000007;
const int INF = INT_MAX / 2;
const ll LINF = LLONG_MAX / 2;

template <typename T1, typename T2>
bool chmax(T1 &a, const T2 &b) {
  if(a < b) {a = b; return true;}
  return false;
}

template <typename T1, typename T2>
bool chmin(T1 &a, const T2 &b) {
  if(a > b) {a = b; return true;}
  return false;
}

template<typename T1, typename T2>
ostream& operator<<(ostream &os, const pair<T1, T2> p) {
  os << p.first << ":" << p.second;
  return os;
}

template<class T>
ostream &operator<<(ostream &os, const vector<T> &v) {
  REP(i, SZ(v)) {
    if(i) os << " ";
    os << v[i];
  }
  return os;
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);

  string s; cin >> s;
  int n = SZ(s);
  vvi nxt(n+1, vi(26, n+1));
  RREP(i, n-1) {
    REP(j, 26) nxt[i][j] = nxt[i+1][j];
    nxt[i][s[i]-'a'] = i+1;
  }

  vector<pair<int, char>> p(n+2);
  vi len(n+2);
  len[n+1] = 0;
  p[n+1] = {INF, '*'};
  RREP(i, n) {
    int mi = INF;
    char arg = 'A';
    int to = INF;
    REP(j, 26) {
      int now = len[nxt[i][j]] + 1;
      if(mi > now) {
        mi = now;
        arg = 'a' + j;
        to = nxt[i][j];
      }
    }
    len[i] = mi;
    p[i] = {to, arg};
  }

  string ans = "";
  int pos = 0;
  while(pos < n+1) {
    ans.push_back(p[pos].second);
    pos = p[pos].first;
  }
  cout << ans << endl;

}