///////////////////////////////////////////////////////////////////////////////

#include <bits/stdc++.h>
#include <unistd.h>
#include <time.h>
#include <sys/time.h>

using namespace std;

///////////////////////////////////////////////////////////////////////////////

#define DEBUG 0

#define pb push_back
#define V vector
#define M unordered_map
#define S static

#define rep(i,n) for(ll i=0LL; i<n; ++i)
#define srep(i,s,n) for(ll i=s; i<n; ++i)
#define rrep(i,n) for(ll i=n-1LL; i>=0LL; --i)
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define CIN(x) do { \
        assert(!cin.eof()); \
        cin >> x; \
        assert(!cin.fail()); \
} while(0);
#if DEBUG
#define debug_print(...) _debug_print(__VA_ARGS__)
#define debug_printf(...) printf(__VA_ARGS__)
#define debug_print_time _debug_print_time
#else // DEBUG
#define debug_print(...)
#define debug_printf(...)
#define debug_print_time
#endif // DEBUG

typedef long long ll;
typedef unsigned long long ull;
typedef tuple<ll, ll> t2;
typedef tuple<ll, ll, ll> t3;
typedef tuple<ll, ll, ll, ll> t4;
typedef tuple<ll, ll, ll, ll, ll> t5;
template <typename T>
using priority_queue_incr = priority_queue<T, V<T>, greater<T> >;

///////////////////////////////////////////////////////////////////////////////

template <typename TT, typename T>
T get_m(M<TT, T> &m, TT k, T default_value)
{
    if (m.find(k) == m.end()) return m[k] = default_value;
    return m[k];
}

template <typename TT, typename T>
void incr_m(M<TT, T> &m, TT k)
{
    if (m.find(k) == m.end()) m[k] = 0;
    m[k]++;
}

template <typename TT, typename T>
void incr_m(map<TT, T> &m, TT k)
{
    if (m.find(k) == m.end()) m[k] = 0;
    m[k]++;
}

void llin(ll &a)
{
    CIN(a);
}

void llinl1(V<ll> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a;
        CIN(a);
        v.push_back(a);
    }
}

void llinl2(V<t2> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b;
        CIN(a >> b);
        v.push_back(t2(a, b));
    }
}

void llinl3(V<t3> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b, c;
        CIN(a >> b >> c);
        v.push_back(t3(a, b, c));
    }
}

void llinl4(V<t4> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b, c, d;
        CIN(a >> b >> c >> d);
        v.push_back(t4(a, b, c, d));
    }
}

void llina(V<ll> &v, ll count)
{
    llinl1(v, count);
}

template <typename T>
void sort(V<T> &v)
{
    sort(v.begin(), v.end());
}

template <typename T>
void sort_reverse(V<T> &v)
{
    sort(v.begin(), v.end(), greater<T>());
}

template <typename T>
void _debug_print(T x)
{
    cout << x << endl;
}

void _debug_print(const t2 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    cout << "-- " << x1 << " -- " << x2 << endl;
}

void _debug_print(const t3 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    ll x3 = get<2>(x);
    cout << "-- " << x1 << " -- " << x2 << " -- " << x3 << endl;
}

void _debug_print(const t4 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    ll x3 = get<2>(x);
    ll x4 = get<3>(x);
    cout << "-- " << x1 << " -- " << x2
         << " -- " << x3 << " -- " << x4 << endl;
}

template <typename T>
void _debug_print(T xarray[], ll n)
{
    rep (i, n) _debug_print(xarray[i]);
}

template <typename T>
void _debug_print(const V<T> &xlist)
{
    for (auto x : xlist) {
        cout << "-- ";
        _debug_print(x);
    }
}

template <typename T>
void _debug_print(const set<T> &xset)
{
    for (auto x : xset) {
        cout << "-- ";
        _debug_print(x);
    }
}

template <typename TT, typename T>
void _debug_print(const M<TT, T> &xlist)
{
    for (auto x : xlist) {
        TT k = x.first;
        T v = x.second;
        cout << "====" << endl;
        cout << "K=";
        _debug_print(k);
        cout << "V=";
        _debug_print(v);
    }
}

template <typename TT, typename T>
void _debug_print(const map<TT, T> &xlist)
{
    for (auto x : xlist) {
        TT k = x.first;
        T v = x.second;
        cout << "====" << endl;
        cout << "K=";
        _debug_print(k);
        cout << "V=";
        _debug_print(v);
    }
}

void _debug_print_time(const char *prefix)
{
    struct timeval tv;
    gettimeofday(&tv, NULL);
#if DEBUG
    struct tm *tm = localtime(&tv.tv_sec);
#endif
    debug_printf("-- %s %02d:%02d:%02d.%06ld\n",
                  prefix, tm->tm_hour, tm->tm_min, tm->tm_sec, tv.tv_usec);
}

///////////////////////////////////////////////////////////////////////////////

void _main();
int main()
{
    cout << setprecision(12);
#if !DEBUG
    ios::sync_with_stdio(false);
    cin.tie(0);
#endif

    _main();
    return 0;
}

#define PHCNT (26)
// #define PHCNT (4)

void _main()
{
    string s; CIN(s);
    ll n = s.size();

    ll cnts[PHCNT] = {};
    ll anslen = 1;
    for (auto c : s) {
        cnts[c-'a']++;
        bool done = true;
        rep (i, PHCNT) if (!cnts[i]) done = false;
        if (done) {
            memset(cnts, 0, sizeof(cnts));
            anslen++;
        }
    }

    debug_printf("---- anslen=%lld\n", anslen);

    memset(cnts, 0, sizeof(cnts));
    S ll remlens[200010];
    ll remlen = 1;
    rrep (i, n) {
        remlens[i] = remlen;
        char c = s[i];
        cnts[c-'a']++;
        bool done = true;
        rep (i, PHCNT) if (!cnts[i]) done = false;
        if (done) {
            memset(cnts, 0, sizeof(cnts));
            remlen++;
        }
    }

    debug_printf("---- remlens\n");
    debug_print(remlens, n+1);

    S ll starts[200010];
    rep (i, 200010) starts[i] = 1e18;
    S ll ends[200010];
    rep (i, 200010) ends[i] = -1e18;
    rep (i, n) {
        ll x = remlens[i];
        starts[x] = min(starts[x], i);
        ends[x] = max(ends[x], i);
    }
    rep (i, 200010) ends[i]++;

    debug_printf("---- starts\n");
    debug_print(starts, 5);
    debug_printf("---- ends\n");
    debug_print(ends, 5);

    string ans;
    ll nowidx = 0;
    ll nowlen = anslen;
    while (nowlen > 1) {
        nowlen--;
        bool ngs[PHCNT] = {};
        srep (i, nowidx, starts[nowlen]) ngs[s[i]-'a'] = true;
        char okc = 'z'+1;
        ll okidx = 1e18;
        srep (i, starts[nowlen], ends[nowlen]) {
            if (ngs[s[i]-'a']) continue;
            if (s[i] < okc) {
                okc = s[i];
                okidx = i;
            }
        }
        debug_printf("okc=%c okidx=%lld\n", okc, okidx);
        ans += okc;
        nowidx = okidx + 1;
    }

    debug_printf("nowidx=%lld\n", nowidx);

    bool last_ngs[PHCNT] = {};
    srep (i, nowidx, ends[1]) last_ngs[s[i]-'a'] = true;
    srep (c, 'a', 'a'+PHCNT) if (!last_ngs[c-'a']) {
        debug_printf("last c = %c\n", (char)c);
        ans += (char)c;
        break;
    }

    debug_printf("---- ans\n");
    cout << ans << endl;
}

///////////////////////////////////////////////////////////////////////////////


Measured memory_bytes: 8440