#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000001


int main(){
	
	string s;
	cin>>s;
	
	vector dp(s.size()+1,vector<int>(27,-Inf));
	dp.back()[26] = 0;
	
	for(int i=s.size()-1;i>=0;i--){
		dp[i] = dp[i+1];
		if(islower(s[i])){
			int c = s[i] - 'a';
			for(int j=c+1;j<27;j++){
				dp[i][c] = max(dp[i][c],dp[i+1][j] + 1);
			}
		}
	}
	

	
	int maxi = 0;
	rep(i,27){
		maxi = max(maxi,dp[0][i]);
	}
	
	if(maxi==0)maxi++;
	
	vector dp2(s.size()+1,vector<int>(27,-Inf));
	dp2[0][0] = 0;
	string ans = "";
	
	rep(i,s.size()){
		int ind = -1;
		for(int j=1;j<27;j++){
			bool f = true;
			rep(k,j){
				for(int l=j;l<27;l++){
					if(dp2[i][k] + dp[i+1][l] + 1 > maxi){
						f = false;
						break;
					}
				}
				if(!f)break;
			}
			if(f){
				ind = j-1;
				break;
			}
					
		}
		
		if(islower(s[i])){
			ind = s[i] - 'a';
		}
		
		ans += 'a' + ind;
		dp2[i+1] = dp2[i];
		/*
		rep(j,5){
			cout<<dp[i+1][j]<<',';
		}
		cout<<endl;*/
		for(int j=0;j<=ind;j++){
			dp2[i+1][ind+1] = max(dp2[i+1][ind+1],dp2[i][j]+1);
		}
		
		
	}
	
	cout<<ans<<endl;
	
	return 0;
}