#include <algorithm>
#include <bitset>
#include <cmath>
#include <complex>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <regex>
#include <set>
#include <stack>
#include <string>
#include <vector>

const int MOD = 1e9 + 7;
const int iINF = 2147483647 / 2;
const long long int llINF = 9223372036854775807 / 2;

using namespace std;
using ll = long long int;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using vvvl = vector<vector<vector<ll>>>;

typedef pair<ll, ll> pll;
bool paircomp(const pll &a, const pll &b) {
  if (a.first == b.first)
    return a.second < b.second;
  return a.first < b.first;
}

#define REP(i, n) for (ll i = 0; i < (n); i++)
#define RREP(i, n) for (ll i = (n)-1; i >= 0; i--)
#define FOR(i, a, b) for (ll i = (a); i < (b); i++)
#define AUTO(i, m) for (auto &i : m)
#define ALL(a) (a).begin(), (a).end()
#define MAX(vec) *std::max_element(vec.begin(), vec.end())
#define MIN(vec) *std::min_element(vec.begin(), vec.end())
#define ARGMAX(vec)                                                            \
  std::distance(vec.begin(), std::max_element(vec.begin(), vec.end()))
#define ARGMIN(vec)                                                            \
  std::distance(vec.begin(), std::min_element(vec.begin(), vec.end()))
#define REV(T) greater<T>()
#define PQ(T) priority_queue<T, vector<T>, greater<T>>
#define VVL(a, b, c) vector<vector<ll>>(a, vector<ll>(b, c))
#define VV(T, a, b, c) vector<vector<T>>(a, vector<T>(b, c))
#define VVVL(a, b, c, d)                                                       \
  vector<vector<vector<ll>>>(a, vector<vector<ll>>(b, vector<ll>(c, d)))
#define VVV(T, a, b, c, d)                                                     \
  vector<vector<vector<T>>>(a, vector<vector<T>>(b, vector<T>(c, d)))
#define SP(a) setprecision(a)
#define SQRT(a) sqrt((long double)(a))
#define DPOW(a, b) pow((long double)(a), (long double)(b))
#define UNIQUE(vec)                                                            \
  do {                                                                         \
    sort(ALL((vec)));                                                          \
    (vec).erase(std::unique(ALL((vec))), (vec).end());                         \
  } while (0)

ll POW(ll n, ll m) {
  if (m == 0) {
    return 1;
  } else if (m % 2 == 0) {
    ll tmp = POW(n, m / 2);
    return (tmp * tmp);
  } else {
    return (n * POW(n, m - 1));
  }
}

int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

bool comp(string a, string b) {
  if (a == "")
    return true;
  if (a.size() > b.size())
    return true;
  else if (a.size() < b.size())
    return false;
  else {
    ll n = a.size();
    REP(i, n) {
      if (a[i] > b[i])
        return true;
      else if (a[i] < b[i])
        return false;
    }
  }
  return false;
}

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  string A;
  cin >> A;
  ll N = A.size();
  vvl POS(26);
  REP(i, N) POS[A[i] - 'a'].push_back(i);

  string ans = "";
  vl DP(N + 3, llINF);
  map<ll, pair<char, ll>> restore;
  DP[N] = 1;
  RREP(i, N) {
    REP(c, 26) {
      ll p = lower_bound(POS[c].begin(), POS[c].end(), i) - POS[c].begin();
      if (p == POS[c].size()) {
        if (DP[i] > 1) {
          restore[i] = {(char)('a' + c), N};
          DP[i] = 1;
        }
        continue;
      }
      ll nxt = POS[c][p];
      if (DP[i] > DP[nxt + 1] + 1) {
        restore[i] = {(char)('a' + c), nxt + 1};
        DP[i] = DP[nxt + 1] + 1;
      }
    }
  }
  ll cur = 0;
  while (cur < N) {
    auto lp = restore[cur];
    ans += lp.first;
    cur = lp.second;
  }
  cout << ans << endl;

  return 0;
}


Measured memory_bytes: 16148