#include<bits/stdc++.h>
using namespace std;
char s[200005];int n;
int dp[200005];int ok[30],sum = 0;
int nxt[200005][26];
vector<int>pos[26];
int rk[200005],sa[200005],tp[200005],a[200005];
int m,k;
void qsort(){
	for(int i = 1;i<=m;i++)a[i] =0;
	for(int i = 1;i<=n;i++)++a[rk[i]];
	for(int i = 1;i<=m;i++)a[i]+=a[i-1];
	for(int i = n;i>=1;i--)sa[a[rk[tp[i]]]--] = tp[i];
	return;
}void construct(){
//	for(int i = 1;i<=n;i++)cout << sa[i] << " ";cout << endl;
	for(int k = 1,p = 0;k<=n;m =p,p = 0,k*=2){
		for(int i = n-k+1;i<=n;i++)tp[++p] = i;
		for(int i = 1;i<=n;i++)if(sa[i]>k)tp[++p] =sa[i]-k;
		qsort();swap(tp,rk);p = rk[sa[1]] = 1;
		for(int i = 2;i<=n;i++){
			rk[sa[i]] = (tp[sa[i]] == tp[sa[i-1]] and tp[sa[i]+k] == tp[sa[i-1]+k])?p:++p;
		}
	}
}void dfs(int now,int nd){
//	cout << now << " " << nd << endl;
	if(now == n+1)return;
	for(int i =0;i<26;i++){
	//	cout << " " << i << " " << nxt[now][i] << " " << dp[nxt[now][i]] << endl;
		if(dp[nxt[now][i]+1]+1 == nd){
			cout << (char)(i+'a');dfs(nxt[now][i]+1,nd-1);return;
		}
	}
}
signed main(){
	scanf("%s",s+1);n = strlen(s+1);
	//cout << n << endl;
	dp[n+1] = 1;
	for(int i = n;i>=1;i--){
		if(ok[s[i]-'a'] == 0)ok[s[i]-'a'] = 1,sum++;
		dp[i] = dp[i+1]+(sum == 26);
		if(sum == 26)sum =0,memset(ok,0,sizeof(ok));
	//	cout << dp[i] << " ";
	}for(int i =0;i<26;i++)pos[i].push_back(0);
	for(int i = 1;i<=n;i++)pos[s[i]-'a'].push_back(i);
	for(int i = 0;i<26;i++)pos[i].push_back(n+1);
	for(int i =0;i<26;i++){
		for(int j = 0;j+1<pos[i].size();j++){
			for(int k =pos[i][j]+1;k<=min(pos[i][j+1],n);k++){
				nxt[k][i] = pos[i][j+1];
			}
		}
	}dfs(1,dp[1]);
	return 0;
}