/*
                                                                                                     
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            .:ooo+++++osso-`    `.:-...`/` ./::-------:/:`   -`         :+--..``````.--:+:...-+:-`  
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                                          `-+o+/:---:/+o/.                                          
                                            `.:+oooo+/-.`                                           
                                               ``````                                               
*/


#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <map>
#include <set>
#include <queue>
#include <ostream>
#include <istream>
#include <typeinfo>
#include <iomanip>
#include <cstdio>
#include <cstdlib>
#include <cassert>
#include <limits>
#include <fstream>
#include <array>
#include <list>
#include <bitset>
#include <functional>

#define mt make_tuple
#define x first
#define y second
#define pb push_back
#define ppb pop_back
#define pf push_front
#define ppf pop_front
#define mp make_pair
#define umap unordered_map
#define uset unordered_set
#define rt return 0;
#define elif else if
#define len(v) ((int)v.size())


using namespace std;

char string_in_buffer[(int)1e6];


void fast_scan(int &x) { scanf("%d", &x); }
void fast_scan(long long &x) { scanf("%lld", &x); }
void fast_scan(unsigned long long &x) { scanf("%llu", &x); }
void fast_scan(double &x) { scanf("%lf", &x); }
void fast_scan(long double &x) { scanf("%Lf", &x); }
void fast_scan(char &x) { 
	scanf("%c", &x); 
	if (x == '\n') {
		fast_scan(x);
	}
}
void fast_scan(string &x) {
	scanf("%s", string_in_buffer);
	x = string(string_in_buffer);
}

template<class TFirst, class TSecond>
void fast_scan(pair<TFirst, TSecond> &p) {
	fast_scan(p.first);
	fast_scan(p.second);
}

template <class T>
void fast_scan(vector<T> &v) {
	for (auto &x : v) fast_scan(x);
}

void fast_print(const int &x) { printf("%d", x); }
void fast_print(const long long &x) { printf("%lld", x); }
void fast_print(const unsigned long long &x) { printf("%llu", x); }
void fast_print(const double &x) { printf("%.15lf", x); }
void fast_print(const long double &x) { printf("%.15Lf", x); }
void fast_print(const char &x) { printf("%c", x); };
void fast_print(const string &x) { printf("%s", x.c_str());}

template<class TFirst, class TSecond>
void fast_print(const pair<TFirst, TSecond> &p) {
	fast_print(p.first);
	fast_print(' ');
	fast_print(p.second);
}

template <class T>
void fast_print(const vector<T> &v) {
	if (v.empty()) return;
	fast_print(v[0]);
	for (int i = 1; i < v.size(); i++) {
		fast_print(" ");
		fast_print(v[i]);
	}
}

template <class T>
void fast_print(const vector<vector<T>> &v) {
	if (v.empty()) return;
	fast_print(v[0]);
	for (int i = 1; i < v.size(); i++) {
		fast_print("\n");
		fast_print(v[i]);
	}
}



using namespace std;


namespace smart_io {
	string print_start = "";
	string sep = " ";
	bool first_print = false;

	void precall_print() {
		fast_print(print_start);
		print_start = "\n";
		first_print = true;
	}
} //namespace smart_io


template <class T>
ostream &operator,(ostream &os, const T &object) {
	if (!smart_io::first_print) {
		fast_print(smart_io::sep);
	} else {
		smart_io::first_print = false;
	}
	fast_print(object);
	return os;
}

template <class T>
istream &operator,(istream &is, T &object) {
	fast_scan(object);
	return is;
}
#define int long long
namespace typedefs {
	typedef long long ll;
	typedef unsigned long long ull;
	typedef pair<int, int> pii;
	typedef long double ld;
}

namespace numbers_operation {
	template<class T>
	T floor_mod(T a, T b) {
		if (a % b == 0) return 0;
		if (a >= 0 && b >= 0) return a % b;
		if (a <= 0 && b <= 0) return a % b;
		return abs(b) - (abs(a) % abs(b));
	}
}

using namespace numbers_operation;
using namespace typedefs;

#define print    \
smart_io::precall_print(); \
cout,

#define scan cin,

int n;
vector<int> v;
vector<vector<int>> fright;
vector<int> dp;

int f(int i) {
	if (i == len(v)) return 1e9;
	if (dp[i] != -1) return dp[i];
	int _min = 1e9;
	for (int j = 0; j < 26; j++) {
		if (fright[i][j] == -1) {
			_min = 1;
		} else {
			_min = min(_min, f(fright[i][j] + 1) + 1);
		}
	}
	return dp[i] = _min;
}

string rez = "";
void backtothe80s(int i) {
	if (i == len(v)) return;
	int _min = 1e9;
	char c = '!';
	for (int j = 0; j < 26; j++) {
		if (fright[i][j] == -1) {
			rez += 'a' + j;
			return;
		}
		if (f(fright[i][j] + 1) + 1 < _min) {
			_min = f(fright[i][j] + 1) + 1;
			c = j + 'a';
		}
	}
	rez += c;
	backtothe80s(fright[i][c - 'a'] + 1);
}

signed main(signed argc, char *argv[]) {
	string s;
	scan s;
	int n = len(s);
	fright.resize(n);
	dp.resize(n, -1);
	for (int i = 0; i < len(s); i++) {
		v.pb(s[i] - 'a');
	}

	vector<int> _right(26, -1);
	for (int i = len(s) - 1; i > -1; i--) {
		_right[v[i]] = i;
		fright[i] = _right;
	}
	backtothe80s(0);
	print rez;

}

Measured memory_bytes: 52592