#include <bits/stdc++.h>
using namespace std;
#define int long long
const int N=1e6+5;
string s;
map<string,int>q;
signed main(){
    cin>>s;
    for(int i=0;i<s.size();i++){
        string t;
        t+=s[i];
        q[t]++;
    }
    while(q.size()>1){
        auto a=*q.begin(),b=*(--q.end());
        q.erase(q.begin());q.erase(--q.end());
        if(a.second>b.second){
            q[a.first+b.first]=b.second;
            q[a.first]=a.second-b.second;
        }else if(a.second==b.second){
            q[a.first+b.first]=a.second;
        }else{
            q[a.first+b.first]=a.second;
            q[b.first]=b.second-a.second;
        }
    }
    auto x=*q.begin();
    while(x.second--)cout<<x.first;
}