#include <bits/stdc++.h>

using namespace std;
#define FOR(i,n) for(int i = 0; i < (n); i++)
#define sz(c) ((int)c.size())
#define ten(n) ((int)1e##n)
using ll = long long;
using Pii = pair<int, int>;
using Pll = pair<ll, ll>;

template<typename ...> static inline int getchar_unlocked(void) { return getchar(); }
template<typename ...> static inline void putchar_unlocked(int c) { putchar(c); }
#define mygc(c) (c)=getchar_unlocked()
#define mypc(c) putchar_unlocked(c)
void reader(int& x) { int k, m = 0; x = 0; for (;;) { mygc(k); if (k == '-') { m = 1; break; }if ('0' <= k&&k <= '9') { x = k - '0'; break; } }for (;;) { mygc(k); if (k<'0' || k>'9')break; x = x * 10 + k - '0'; }if (m) x = -x; }
void reader(ll& x) { int k, m = 0; x = 0; for (;;) { mygc(k); if (k == '-') { m = 1; break; }if ('0' <= k&&k <= '9') { x = k - '0'; break; } }for (;;) { mygc(k); if (k<'0' || k>'9')break; x = x * 10 + k - '0'; }if (m) x = -x; }
int reader(char c[]) { int i, s = 0; for (;;) { mygc(i); if (i != ' '&&i != '\n'&&i != '\r'&&i != '\t'&&i != EOF) break; }c[s++] = i; for (;;) { mygc(i); if (i == ' ' || i == '\n' || i == '\r' || i == '\t' || i == EOF) break; c[s++] = i; }c[s] = '\0'; return s; }
int reader(string& c) { int i; for (;;) { mygc(i); if (i != ' '&&i != '\n'&&i != '\r'&&i != '\t'&&i != EOF) break; }c.push_back(i); for (;;) { mygc(i); if (i == ' ' || i == '\n' || i == '\r' || i == '\t' || i == EOF) break; c.push_back(i); }; return sz(c); }
template <class T, class S> void reader(T& x, S& y) { reader(x); reader(y); }
template <class T, class S, class U> void reader(T& x, S& y, U& z) { reader(x); reader(y); reader(z); }
template <class T, class S, class U, class V> void reader(T& x, S& y, U& z, V & w) { reader(x); reader(y); reader(z); reader(w); }
void writer(int x, char c) { int s = 0, m = 0; char f[10]; if (x<0)m = 1, x = -x; while (x)f[s++] = x % 10, x /= 10; if (!s)f[s++] = 0; if (m)mypc('-'); while (s--)mypc(f[s] + '0'); mypc(c); }
void writer(ll x, char c) { int s = 0, m = 0; char f[20]; if (x<0)m = 1, x = -x; while (x)f[s++] = x % 10, x /= 10; if (!s)f[s++] = 0; if (m)mypc('-'); while (s--)mypc(f[s] + '0'); mypc(c); }
void writer(const char c[]) { int i; for (i = 0; c[i] != '\0'; i++)mypc(c[i]); }
void writer(const string& x, char c) { int i; for (i = 0; x[i] != '\0'; i++)mypc(x[i]); mypc(c); }
void writer(const char x[], char c) { int i; for (i = 0; x[i] != '\0'; i++)mypc(x[i]); mypc(c); }
template<class T> void writerLn(T x) { writer(x, '\n'); }
template<class T, class S> void writerLn(T x, S y) { writer(x, ' '); writer(y, '\n'); }
template<class T, class S, class U> void writerLn(T x, S y, U z) { writer(x, ' '); writer(y, ' '); writer(z, '\n'); }
template<class T> void writerArr(T x[], int n) { if (!n) { mypc('\n'); return; }FOR(i, n - 1)writer(x[i], ' '); writer(x[n - 1], '\n'); }
template<class T> void writerArr(vector<T>& x) { writerArr(x.data(), (int)x.size()); }

template<class T> void chmin(T& a, const T& b) { if (a > b) a = b; }
template<class T> void chmax(T& a, const T& b) { if (a < b) a = b; }

template<class T> T gcd(T a, T b) { return b ? gcd(b, a % b) : a; }
template<class T> T lcm(T a, T b) { return a / gcd(a, b) * b; }
ll mod_pow(ll a, ll n, ll mod) {
	ll ret = 1;
	ll p = a % mod;
	while (n) {
		if (n & 1) ret = ret * p % mod;
		p = p * p % mod;
		n >>= 1;
	}
	return ret;
}
template<class T> T extgcd(T a, T b, T& x, T& y) { for (T u = y = 1, v = x = 0; a;) { T q = b / a; swap(x -= q * u, u); swap(y -= q * v, v); swap(b -= q * a, a); } return b; }
template<class T> T mod_inv(T a, T m) { T x, y; extgcd(a, m, x, y); return (m + x % m) % m; }

const int MOD = ten(9) + 7;

bool chmin(int& a, int b) {
	if (a > b) {
		a = b;
		return true;
	}
	return false;
}

bool chmin(Pii& a, Pii b) {
	if (a > b) {
		a = b;
		return true;
	}
	return false;
}


int dp[ten(5) * 2 + 10][26];
vector<int> nidx[26];

int main() {
	memset(dp, 0x2f, sizeof(dp));
	FOR(i, 26) dp[0][i] = 1;

	string s; reader(s);
	FOR(i, sz(s)) {
		nidx[s[i] -'a'].push_back(i);
	}

	reverse(s.begin(), s.end());
	FOR(i, sz(s)) {
		FOR(j, 26) FOR(k, 26) {
			int nxt;
			if (j == k) {
				if (s[i] == 'a' + j) {
					nxt = dp[i][j] + 1;
				} else {
					nxt = dp[i][j];
				}
			} else {
				nxt = dp[i][j] + 1;
			}
			chmin(dp[i + 1][k], nxt);
		}
	}

	int mn = sz(s);
	FOR(i, 26) {
		chmin(mn, dp[sz(s)][i]);
	}

	string ans;
	int cur = 0;
	while (cur <= sz(s)) {
		bool brk = false;

		FOR(i, 26) {
			auto it = lower_bound(nidx[i].begin(), nidx[i].end(), cur);
			if (it == nidx[i].end()) {
				ans.push_back('a' + i);
				cur = sz(s) + 1;
				break;
			}
			int ncur = *it + 1;

			int mn2 = ten(5) * 2;
			FOR(i, 26) {
				chmin(mn2, dp[sz(s) - ncur][i]);
			}
			if (sz(ans) + mn2 + 1 == mn) {
				ans.push_back('a' + i);
				cur = ncur;
				break;
			}
		}
	}
	cout << ans << endl;

	return 0;
}

Measured memory_bytes: 21764