#include<bits/stdc++.h>
using namespace std;
namespace my_std{
	#define ll long long
	#define bl bool
	ll my_pow(ll a,ll b,ll mod){
		ll res=1;
		if(!b) return 1;
		while(b){
			if(b&1) res=(res*a)%mod;
			a=(a*a)%mod;
			b>>=1;
		}
		return res;
	}
	ll qpow(ll a,ll b){
		ll res=1;
		if(!b) return 1;
		while(b){
			if(b&1) res*=a;
			a*=a;
			b>>=1;
		}
		return res;
	}
	#define db double
	#define pf printf
	#define pc putchar
	#define fr(i,x,y) for(register ll i=(x);i<=(y);i++)
	#define pfr(i,x,y) for(register ll i=(x);i>=(y);i--)
	#define go(u) for(ll i=head[u];i;i=e[i].nxt)
	#define enter pc('\n')
	#define space pc(' ')
	#define fir first
	#define sec second
	#define MP make_pair
	#define il inline
	#define inf 8e18
	#define random(x) rand()*rand()%(x)
	#define inv(a,mod) my_pow((a),(mod-2),(mod))
	il ll read(){
		ll sum=0,f=1;
		char ch=0;
		while(!isdigit(ch)){
			if(ch=='-') f=-1;
			ch=getchar();
		}
		while(isdigit(ch)){
			sum=sum*10+(ch^48);
			ch=getchar();
		}
		return sum*f;
	}
	il void write(ll x){
		if(x<0){
			x=-x;
			pc('-');
		}
		if(x>9) write(x/10);
		pc(x%10+'0');
	}
	il void writeln(ll x){
		write(x);
		enter;
	}
	il void writesp(ll x){
		write(x);
		space;
	}
}
using namespace my_std;
vector<ll> vec[1000010];
ll n,fa[1000010],pre[1000010];
char s[1000010];
void dfs(ll x){
	pc(s[x]);
	fr(i,0,(ll)vec[x].size()-1) dfs(vec[x][i]);
}
int main(){
	scanf("%s",s+1);
	n=strlen(s+1);
	sort(s+1,s+n+1);
	if(s[1]==s[n]){
		fr(i,1,n) pc(s[i]);
		return 0;
	}
	ll now=n,r=n;
	while(s[now]!=s[1]) now--;
	fr(i,2,n) if(s[i-1]==s[i]&&s[i]!=s[1]) pre[i]=i-1;
	while(r>1){
		ll l=r;
		while(pre[l]) l=pre[l];
		now=min(now,l-1);
		ll tmp=1;
		pfr(i,r,l){
			if(tmp==1) tmp=now;
			else tmp--;
			fa[i]=tmp;
		}
		if(tmp!=1){
			fr(i,tmp+1,now) pre[i]=i-1;
			now=tmp-1;
		}
		r=l-1;
	}
	pfr(i,n,2) vec[fa[i]].push_back(i);
	dfs(1);
}