#define STOPIT
#include <bits/stdc++.h>
#define loop(n) for (lint ngtkana_is_a_genius = 0; ngtkana_is_a_genius < lint(n); ngtkana_is_a_genius++)
#define rep(i, begin, end) for (lint i = lint(begin); (i) < lint(end); i++)
#define all(v) v.begin(), v.end()
#define rand(l, r) std::uniform_int_distribution<>(l, r)(mt)
using lint = long long;
auto mt = std::mt19937_64(std::random_device{}());
auto cmn = [](auto& a, auto b){ if (a > b) {a = b; return true;} return false; };
auto cmx = [](auto& a, auto b){ if (a < b) {a = b; return true;} return false; };
void debug_impl() { std::cerr << std::endl; }
template <typename Head, typename... Tail>
void debug_impl(Head head, Tail... tail) { std::cerr << " " << head; debug_impl(tail...); }
#ifndef STOPIT
#define debug(...)\
	do {\
		std::cerr << std::boolalpha << "[" << #__VA_ARGS__ << "]:";\
		debug_impl(__VA_ARGS__);\
		std::cerr << std::noboolalpha;\
	} while (false)
#else
#define debug(...) {}
#endif

template < typename Container, typename Value = typename Container::value_type, std::enable_if_t<!std::is_same< Container, std::string >::value, std::nullptr_t> = nullptr>
std::istream& operator>> (std::istream& is, Container& v)
	{ for (auto & x : v) { is >> x; } return is; }

template < typename Container, typename Value = typename Container::value_type, std::enable_if_t<!std::is_same< Container, std::string >::value, std::nullptr_t> = nullptr >
std::ostream& operator<< (std::ostream& os, Container const& v) {
 os << "{";
	for (auto it = v.begin(); it != v.end(); it++)
		{os << (it != v.begin() ? "," : "") << *it;}
	return os << "}";
}

template <typename T>
auto make_vector_impl(size_t sz, T t) {return std::vector<T>(sz, t);}

template <size_t N, typename T, typename U, std::enable_if_t<N == 1, std::nullptr_t> = nullptr>
auto make_vector(size_t sz, U u) {return make_vector_impl(sz, T(u));}

template <size_t N, typename T, std::enable_if_t<N == 1, std::nullptr_t> = nullptr>
auto make_vector(size_t sz) {return std::vector<T>(sz);}

template <size_t N, typename T, typename... Args, std::enable_if_t<N != 1, std::nullptr_t> = nullptr>
auto make_vector(size_t a, Args... args) {return make_vector_impl(a, make_vector<N - 1, T>(args...));}

template <typename T, typename Size_t>
auto& at(T& t, Size_t i) {return t.at(i);}

template <typename T, typename Size_t, typename... Args>
auto& at(T& t, Size_t i, Args... args) {return at(t.at(i), args...);}

// require gcc >= 7.1.0
template < class ... Keys >
class fmt_t {
		using storage_t = std::tuple< Keys ... >;
		storage_t storage;
		static constexpr std::size_t N = sizeof ... (Keys);

		// is_exact_match_impl
		template < std::size_t I, class Value >
			auto is_exact_match_impl(long, Value) const
				{ return false; }

		template < std::size_t I, class Value,
			std::enable_if_t<
				std::is_same< std::decay_t< Value > , char >::value
				== std::is_same< std::tuple_element_t< I, storage_t >, char >::value,
			std::nullptr_t > = nullptr,
			class = decltype(std::declval< Value >() == std::get< I >(std::declval< storage_t >())) >
			auto is_exact_match_impl(int, Value x) const
				{ return x == std::get< I >(storage); }

		template < class Value, std::size_t ... Is >
			auto find_exact_match_impl(Value x, std::integer_sequence< std::size_t, Is ... >) const {
				auto ret = false;
				(void)std::initializer_list< std::nullptr_t >
					{ (ret |= is_exact_match_impl< Is >(int{}, x), nullptr) ...  };
				return ret;
			}

		template < class Value >
			auto exists_exact_match(Value x) const
			{  return find_exact_match_impl(x, std::make_index_sequence< N >()); }

		// tuple_format_impl
		template < class Tuple,  std::size_t ... Is >
		std::string tuple_format_impl(const Tuple& tuple, std::integer_sequence< std::size_t, Is ... >) const {
			std::string ret = "(";
			(void)std::initializer_list< std::nullptr_t >
				{ (ret = ret + (Is == 0 ? "" : ",") + format(std::get< Is >(tuple)), nullptr) ... };
			return ret + ")";
		}

		// usual_format
		template < class Value >
		std::string usual_format(long, Value x) const
			{ return std::to_string(x); }

		template < class Value, std::enable_if_t< std::is_same< std::decay_t< Value >, char >::value, std::nullptr_t > = nullptr >
		std::string usual_format(int, Value c) const
			{ return std::string{'\''} + std::string{c} + std::string{'\''}; }

		std::string usual_format(int, char const* s) const
			{ return "\"" + std::string(s) + "\""; }

		std::string usual_format(int, std::string const s) const
			{ return "\"" + s + "\""; }

		template < class Container, class = typename Container::value_type >
		std::string usual_format(int, Container const & v) const {
			return "{" + std::accumulate(v.begin(), v .end(), std::string{},
				[this](auto const& s, auto const x) { return s + (s.empty() ? "" : ",") + this->format(x); })
				+ "}";
		}

		template < class Tuple, std::size_t N = std::tuple_size< Tuple >::value >
		std::string usual_format(int, Tuple const& tuple) const
			{ return tuple_format_impl(tuple, std::make_index_sequence< N >()); }

		// format
		template < class Value > std::string format(Value&& x) const
			{ return exists_exact_match(x) ? std::string{'_'} : usual_format(int{}, x); }

	public:
		fmt_t()=default;
		template < Keys ... > fmt_t(Keys&& ... keys ) : storage(keys ... ){}

		template < class Value > std::string operator()(Value&& x) const
			{ return format(x); }
};
template < class ... Keys > auto fmt(Keys&& ... keys)
	{ return fmt_t< Keys ... >(std::forward< Keys >(keys) ... ); }

int main() {
	std::cin.tie(0); std::cin.sync_with_stdio(false);
	std::string s; std::cin >> s;
	int n = s.length();
	std::vector<int> a(n);
	std::transform(all(s), a.begin(), [](char c){ return c - 'a'; });

	int alpha = 26;
	auto pt = [&] {
		std::vector<int> pt = {n};
		std::vector<int> now(alpha,0);
		int kind = 0;
		for (int i = n - 1; 0 <= i; i--) {
			int x = a.at(i);
			if (!now.at(x)) kind++;
			now.at(x)++;
			if (kind == alpha) {
				pt.emplace_back(i);
				now.assign(alpha, 0);
				kind = 0;
			}
		}
		return pt;
	}();
	std::reverse(all(pt));
	debug(pt);

	int inf = std::numeric_limits<int>::max();
	auto next = [&]	{
		auto next = make_vector<2,int>(alpha,n,inf);
		rep(i,0,alpha) {
			for (int j = n - 2; 0 <= j; j--) {
				int & crr = at(next, i, j);
				int & nxt = at(next, i, j + 1);
				if (j < n - 1) crr = nxt;
				if (a.at(j) == i) crr = j;
			}
		}
		return next;
	}();
	debug(fmt(inf)(next));

	std::vector<int> ans;
	int l = 0;
	if (pt.front() == 0) {
		ans.emplace_back(0);
		l = at(next, 0, 0) + 1;
		pt.erase(pt.begin());
	}

	for (int r : pt) {
		debug(l,r);
		std::vector<int> mul(alpha, 0);
		for (int x : std::vector<int>{a.begin()+l, a.begin()+r}) {
			mul.at(x)++;
		}
		debug(mul);
		int x = -1;
		rep(i,0,alpha) {
			if (!mul.at(i)) {
				x = i;
				break;
			}
		}
		assert(x != -1);
		ans.emplace_back(x);
		if (r == n) break;
		l = at(next, x, r) + 1;
	}
	debug(ans);

	for (int x : ans) {
		char c = x + 'a';
		std::cout << c;
	}
	std::cout << std::endl;
	return 0;
}

Measured memory_bytes: 22548