/*
ID: Sho10
LANG: C++
TASK:
*/
#include <bits/stdc++.h> //JuniorMonster a.k.a Sho10
#define ll long long int
#pragma GCC optimize("O3")
#pragma GCC optimize("Ofast")
#define all(a) (a).begin(), (a).end()
#define sz size
#define f first
#define s second
#define pb push_back
#define er erase
#define in insert
#define mp make_pair
#define pi pair
#define rc(s) return cout<<s,0
#define endl '\n'
#define mod 1000000007
#define PI 3.14159265359
#define CODE_START  ios_base::sync_with_stdio();cin.tie();cout.tie();
using namespace std;
ll n;
string s,a;
map<char,ll>m;
char x='1';
int32_t main(){
CODE_START;
ifstream cin("ordine.in");
ofstream cout("ordine.out");
cin>>a;
n=a.size();
sort(a.begin(),a.end());
for(ll i=0;i<n;i++)
{
    m[a[i]]++;
}
for(ll i=0;i<n;i++)
{
    ll s1=-1;
    if((n-i)%2==1){
        for(char j='a';j<='z';j++)
        if(m[j]>=(n-i)/2+1){
            m[j]--;
            s.pb(j);
            s1=1;
            break;
        }
    }
    if(s1==-1){
            for(char j='a';j<='z';j++)
            {
                if(m[j]>=1&&s[s.size()-1]!=char(j)){
                    s.pb(j);
                    m[j]--;
                    break;
                }
            }
    }
}
cout<<s<<endl;
}