#include <bits/stdc++.h>
using namespace std;

// template {{{
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define mt make_tuple
#define lb lower_bound
#define ub upper_bound
#define f first
#define s second

#define sz(x) int((x).size())
#define all(x) (x).begin(), (x).end()

#define FOR(i, a, b)    for (int i = (a); i < (b); i++)
#define F0R(i, a)       for (int i = 0; i < (a); i++)
#define FORd(i, a, b)   for (int i = (b)-1; i >= (a); i--)
#define F0Rd(i, a)      for (int i = (a)-1; i >= 0; i--)
#define trav(a, x)      for (auto& a : x)

using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vb = vector<bool>;
using vd = vector<double>;
using vs = vector<string>;

template<typename T> void ckmin(T& a, const T& b) { a = min(a, b); }
template<typename T> void ckmax(T& a, const T& b) { a = max(a, b); }

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());

namespace __input {
    template<class T1, class T2> void re(pair<T1,T2>& p);
    template<class T> void re(vector<T>& a);
    template<class T, size_t SZ> void re(array<T,SZ>& a);

    template<class T> void re(T& x) { cin >> x; }
    void re(double& x) { string t; re(t); x = stod(t); }
    template<class Arg, class... Args> void re(Arg& first, Args&... rest) {
        re(first); re(rest...);
    }

    template<class T1, class T2> void re(pair<T1,T2>& p) { re(p.f,p.s); }
    template<class T> void re(vector<T>& a) { F0R(i,sz(a)) re(a[i]); }
    template<class T, size_t SZ> void re(array<T,SZ>& a) { F0R(i,SZ) re(a[i]); }
}
using namespace __input;

namespace __output {
    template<class T1, class T2> void pr(const pair<T1,T2>& x);
    template<class T, size_t SZ> void pr(const array<T,SZ>& x);
    template<class T> void pr(const vector<T>& x);
    template<class T> void pr(const set<T>& x);
    template<class T1, class T2> void pr(const map<T1,T2>& x);

    template<class T> void pr(const T& x) { cout << x; }
    template<class Arg, class... Args> void pr(const Arg& first, const Args&... rest) {
        pr(first); pr(rest...);
    }

    template<class T1, class T2> void pr(const pair<T1,T2>& x) {
        pr("{",x.f,", ",x.s,"}");
    }
    template<class T> void prContain(const T& x) {
        pr("{");
        bool fst = 1; for (const auto& a: x) pr(!fst?", ":"",a), fst = 0;
        pr("}");
    }
    template<class T, size_t SZ> void pr(const array<T,SZ>& x) { prContain(x); }
    template<class T> void pr(const vector<T>& x) { prContain(x); }
    template<class T> void pr(const set<T>& x) { prContain(x); }
    template<class T1, class T2> void pr(const map<T1,T2>& x) { prContain(x); }

    void ps() { pr("\n"); }
    template<class Arg> void ps(const Arg& first) {
        pr(first); ps();
    }
    template<class Arg, class... Args> void ps(const Arg& first, const Args&... rest) {
        pr(first," "); ps(rest...);
    }
}
using namespace __output;
// }}}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);

    string a; re(a); int N = sz(a);

    vvi occ(26);
    F0R (i, N) occ[a[i] - 'a'].pb(i);
    F0R (c, 26) occ[c].pb(N);

    auto ni = [&occ](int c, int i) {
        return *lower_bound(all(occ[c]), i) + 1;
    };

    vector<pair<int, char>> dp(N + 2, mp(INT_MAX, 'a'));
    dp[N + 1] = { 0, 0 };
    dp[N] = { 1, 'a' };

    F0Rd (i, N) {
        F0R (c, 26) {
            ckmin(dp[i], { 1 + dp[ni(c, i)].f, 'a' + c });
        }
    }

    stringstream ans;
    for (int i = 0; i <= N; ) {
        ans << dp[i].s;
        i = ni(dp[i].s - 'a', i);
    }
    ps(ans.str());

    return 0;
}



Measured memory_bytes: 2964