#include<bits/stdc++.h>
#define ll long long
#define N 1000005
using namespace std;
ll n,i,rk[N],head[N],ls[N],rs[N],las[N],temp[N],pos,trk[N],thd[N],tla[N];
pair<ll,ll> tmp[N];
char s[N];
inline bool cmp(ll a,ll b){return tmp[a]<tmp[b];}
inline void merge(ll a,ll b){rs[las[a]]=head[b],ls[head[b]]=las[a],las[a]=las[b];}
inline void solve(ll len){
	if(len==1){
		for(ll i=head[1];i;i=rs[i]) cout<<s[i];
		cout<<endl;
		return ;
	}
	ll k = 1;
	while(k+1<=len&&rk[k+1]==rk[k]) k++;
	if(len-k>=k){
		for(ll i=1;i<=k;i++) merge(i,len-i+1),tmp[i]=make_pair(rk[i],rk[len-i+1]);
		for(ll i=k+1;i<=len-k;i++) tmp[i]=make_pair(rk[i],0);
		len -= k;
	}
	else{
		for(ll i=1;i<=len-k;i++) merge(i,len-i+1),tmp[i]=make_pair(rk[i],rk[len-i+1]);
		for(ll i=len-k+1;i<=k;i++) tmp[i]=make_pair(rk[i],0);
		len = k;
	}
	for(ll i=1;i<=len;i++) temp[i]=i;
	sort(temp+1,temp+len+1,cmp);
	for(ll i=1;i<=len;i++) thd[i]=head[temp[i]],tla[i]=las[temp[i]];
	for(ll i=1,j=1;i<=len;i=j){
		pos++;
		for(j=i;j<=len;j++) if(tmp[temp[i]]!=tmp[temp[j]]) break;
		for(ll k=i;k<j;k++) trk[k]=pos;
	}
	for(ll i=1;i<=len;i++) rk[i]=trk[i],head[i]=thd[i],las[i]=tla[i];
	solve(len);
}
int main(){
	ios::sync_with_stdio(false);
	cin>>(s+1),n=strlen(s+1);
	sort(s+1,s+n+1);
	for(i=1;i<=n;i++) rk[i]=s[i]-'a'+1,head[i]=i,las[i]=i,ls[i]=rs[i]=0;
	solve(n);
	return 0; 
}