#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <stack>
#include <cmath>
#include <deque>
#include <queue>
#include <map>
#include <bitset>
#include <set>
#include <list>
#include <unordered_map>
#include <unordered_set>
#include <sstream>
#include <numeric>
#include <climits>
#include <utility>
#include <iomanip>
#include <cassert>

using namespace std;

using ll = long long;
using ii = pair<int, int>;
using iii = pair<int, ii>;
template <class T>
using vv = vector<vector<T>>;

#define rep(i, b) for (int i = 0; i < int(b); ++i)
#define reps(i, a, b) for (int i = int(a); i < int(b); ++i)
#define rrep(i, b) for (int i = int(b) - 1; i >= 0; --i)
#define rreps(i, a, b) for (int i = int(b) - 1; i >= a; --i)
#define repe(i, b) for (int i = 0; i <= int(b); ++i)
#define repse(i, a, b) for (int i = int(a); i <= int(b); ++i)
#define rrepe(i, b) for (int i = int(b); i >= 0; --i)
#define rrepse(i, a, b) for (int i = int(b); i >= int(a); --i)

#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define sz(a) int(a.size())
#define mp(a, b) make_pair(a, b)

#define inf (INT_MAX / 2)
#define infl (LONG_MAX / 2)
#define infll (LLONG_MAX / 2)

#define X first
#define Y second
#define pb push_back
#define eb emplace_back

// tools for pair<int, int> & graph
template <class T, size_t M, size_t N>
class graph_delegate_t {
    T (&f)[M][N];

public:
    graph_delegate_t(T (&f)[M][N]) : f(f) {}
    T& operator[](const ii& s) { return f[s.first][s.second]; }
    const T& operator[](const ii& s) const { return f[s.first][s.second]; }
};
ii operator+(const ii& lhs, const ii& rhs) {
    return mp(lhs.first + rhs.first, lhs.second + rhs.second);
}

// clang-format off
template <class T> ostream& operator<<(ostream& os, const vector<T>& t) { os << "{"; rep(i, t.size() - 1) { os << t[i] << ","; } if (!t.empty()) os << t.back(); os << "}"; return os; }
template <class S, class T> ostream& operator<<(ostream& os, const pair<S, T>& t) { return os << "(" << t.first << "," << t.second << ")"; }
vector<string> __macro_split(const string& s) { vector<string> v; int d = 0, f = 0; string t; for (char c : s) { if (!d && c == ',') v.pb(t), t = ""; else t += c; if (c == '\"' || c == '\'') f ^= 1; if (!f && c == '(') ++d; if (!f && c == ')') --d; } v.pb(t); return v; }
void __args_output(vector<string>::iterator, vector<string>::iterator) { cerr << endl; }
template <typename T, typename... Args>
void __args_output(vector<string>::iterator it, vector<string>::iterator end, T a, Args... args) { cerr << it->substr((*it)[0] == ' ', it->length()) << " = " << a; if (++it != end) { cerr << ", "; } __args_output(it, end, args...); }
#define out(args...) { vector<string> __args = __macro_split(#args); __args_output(__args.begin(), __args.end(), args); }
// clang-format on

const int MAX_N = 2e5 + 5;

int next_[MAX_N][26];
int dp[MAX_N];

int main() {
    string s;
    cin >> s;
    fill_n(next_[sz(s)], 26, -1);
    rrep(i, sz(s)) {
        dp[i] = INT_MAX;
        rep(j, 26) { next_[i][j] = next_[i + 1][j]; }
        next_[i][s[i] - 'a'] = i;
    }

    rrep(i, sz(s)) {
        rep(j, 26) {
            auto ic = next_[i][j];
            if (ic == -1) {
                dp[i] = 1;
                break;
            } else if (ic + 1 >= sz(s)) {
                dp[i] = min(dp[i], 2);
            } else dp[i] = min(dp[i], dp[ic + 1] + 1);
        }
    }

    string res;
    int i = 0;
    bool end = false;
    while (!end) {
        rep(j, 26) {
            auto ic = next_[i][j];
            if (ic == -1 && dp[i] == 1) {
                res += 'a' + j;
                end = true;
                break;
            } else if (ic + 1 >= sz(s) && dp[i] == 2) {
                res += 'a' + j;
                res += 'a';
                end = true;
                break;
            } else if (dp[i] == dp[ic + 1] + 1){
                res += 'a' + j;
                i = ic + 1;
                break;
            }
        }
    }
    cout << res << endl;
    return 0;
}


Measured memory_bytes: 21636