#include
#include<bits/stdc++.h>
using namespace std;
string recognizes(string s)
{
unordered_map<char,int> mp;
for(int i=0; i<s.size(); i++)
{
mp[s[i]]++;
}
priority_queue<pair<int,char>> q;
for(auto x:mp)
{
q.push({x.second,x.first});
}
string ans="";
while(!q.empty())
{
pair<int,char> temp=q.top();
ans+=temp.second;
temp.first--;
q.pop();
if(!q.empty())
{
pair<int,char> temp2=q.top();
ans+=temp2.second;
temp2.first--;
q.pop();
if(temp2.first>0)
q.push(temp2);
if(temp.first>0)
q.push(temp);
}
else
{
if(temp.first!=0)
return "";
}
}
return ans;
}
int main()
{
string s;
getline(cin,s);
string res=recognizes(s);
cout<<res<<endl;
return 0;
}