#include <bits/stdc++.h>
using namespace std;
vector<int> a[30];
int f[200005], g[200005], n, c[200005];
string s;
inline void print(int p) {
	cout << char(c[p] + 'a' - 1);
	if (f[p] == 1) {
		cout << char(g[p] + 'a' - 1);
		return;
	}
	print(g[p]);
}
signed main() {
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	cin >> s;
	n = s.size();
	for (int i = 0; i < n; i++) {
		c[i + 1] = s[i] - 'a' + 1;
	}
	for (int i = 1; i <= n; i++) {
		a[c[i]].push_back(i);
	}
	for (int i = 1; i <= 26; i++) {
		if (a[i].size() == 0) {
			cout << char('a' + i - 1);
			return 0;
		}
	}
	for (int i = n; i >= 1; i--) {
		f[i] = 0x3f3f3f3f;
		for (int j = 1; j <= 26; j++) {
			if (lower_bound(a[j].begin(), a[j].end(), i + 1) == a[j].end()) {
				f[i] = 1;
				g[i] = j;
				break;
			} else {
				int op = a[j][lower_bound(a[j].begin(), a[j].end(), i + 1) - a[j].begin()];
				if (f[op] + 1 < f[i]) {
					f[i] = f[op] + 1;
					g[i] = op;
				}
			}
		}
	}
	int mina = 0x3f3f3f3f, minb;
	for (int i = 1; i <= 26; i++) {
		if (f[a[i][0]] < mina) {
			mina = f[a[i][0]];
			minb = a[i][0];
		}
	}
	print(minb);
}