//#pragma GCC optimize("Ofast,no-stack-protector")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
#pragma GCC optimize("unroll-loops")
/*#pragma GCC optimize("section-anchors")
#pragma GCC optimize("profile-values,profile-reorder-functions,tracer")
#pragma GCC optimize("vpt")
#pragma GCC optimize("rename-registers")
#pragma GCC optimize("move-loop-invariants")
#pragma GCC optimize("unswitch-loops")
#pragma GCC optimize("function-sections")
#pragma GCC optimize("data-sections")
#pragma GCC optimize("branch-target-load-optimize")
#pragma GCC optimize("branch-target-load-optimize2")
#pragma GCC optimize("btr-bb-exclusive")*/
//#pragma comment(linker, "/STACK:367077216")
#define _CRT_SECURE_NO_WARNINGS
#include <chrono>
#include <set>
#include <map>
#include <deque>
#include <string>
#include <cstdint>
#include <cmath>
#include <queue>
#include <cassert>
#include <random>
#include <bitset>
#include <iomanip>
#include <numeric>
#include <time.h>//////////////
#include <ctime>
#include <string>
#include <cstdio>
#include <vector>
#include <cstdlib>
#include <iostream>
#include <algorithm>
#include <unordered_map>
#include <unordered_set>
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
//#define endl '\n'
#define mp make_pair
#define pbc push_back
#define pob pop_back()
#define empb emplace_back
#define queuel queue<long long>
#define sqr(a) ((a) * (a))
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define pin(p) cin >> p.first >> p.second;
#define uniq(a) sort(all(a));(a).resize(unique(all(a)) - a.begin());
#define rev(v) reverse(v.begin(), v.end());
#define sout(s, c) for (auto i : s) cout << i << c;
#define pout(p) cout << p.first << " " << p.second;
#define er(v, l, r) erase(v.begin() + l, v.begin() + r);
#define vin(v) for (ll i = 0; i < v.size(); ++i) cin >> v[i];
#define vout(v, c) for (int i = 0; i < v.size(); ++i) cout << v[i] << c;
#define pushi(v, a) for (int i = 0; i < a.size(); ++i) v.push_back(a[i]);
#define fastio() ios_base::sync_with_stdio(0); cout.tie(0); cin.tie(0); srand(time(NULL))
#define dab(v) for(auto i:v)cout<<i<<' ';
#define sp system("pause")
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
using namespace std;
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
const ld EPS = 1e-10;
const ld PI = acos(-1);
const int MOD7 = 1000000007;
const int MOD9 = 1000000009;
const int mod = MOD7;
const int inf = 1e9;
signed main()
{
	fastio();
	string s;
	cin >> s;
	int cnt[26];
	fill(cnt, cnt + 26, 0);
	for (auto i : s) cnt[i - 'a']++;
	for (int i = 0; i < 26; ++i)
	{
		if (!cnt[i])
		{
			cout << (char)('a' + i);
			return 0;
		}
	}
	vector<vector<int>> next(s.size(), vector<int>(26));
	vector<int> dp(s.size() + 1, 1);
	for (int i = s.size() - 1; i >=0; --i)
	{
		for (int x = 0; x < 26; ++x)
		{
			next[i][x] = (i + 1 == s.size() ? s.size() : next[i + 1][x]);
		}
		next[i][s[i] - 'a'] = i;
	}
	string ans = "";
	int ti = 0;
	for (int i = s.size() - 1; i >= 0; --i)
	{
		int mx = inf;
		for (int j = 0; j < 26; ++j)
		{
			if (next[i][j] >= s.size()) mx = 0;
			else
				mx = min(mx, dp[next[i][j] + 1]);
		}
		dp[i] = mx + 1;
	}
	//cout << dp[0] << endl;
	while (ti < s.size())
	{
		int mx = inf, mxp = 228;
		for (int j = 0; j < 26; ++j)
		{
			if (next[ti][j] >= s.size())
			{
				mx = 0, mxp = j;
				break;
			}
			if (dp[next[ti][j] + 1] < mx)
			{
				mx = dp[next[ti][j] + 1];
				mxp = j;
			}
		}
		if (mxp == 228) break;
		ti = next[ti][mxp] + 1;
		ans.pbc((char)(mxp + 'a'));
	}
	cout << ans;
	//sp;
}

Measured memory_bytes: 30516