#include <bits/stdc++.h>

using namespace std;
ifstream in("ordine.in");
ofstream out("ordine.out");
std::map<char,int>ap;
int main()
{
    char x;
    int nr=0;
    while(in>>x)
    {nr++;
        ap[x]++;
    }
    bool prim=false;
    char y;
         while(nr--)
         {   if(!prim)
         {
             prim=true;
              for (std::map<char,int>::iterator it=ap.begin(); it!=ap.end(); ++it)
               {y=it->first;it->second--; break;
               }
               out<<y;
         }
         else
              for (std::map<char,int>::iterator it=ap.begin(); it!=ap.end(); ++it)
                { if(it->second && it->first!=y)
                {
    out << it->first; it->second--; y=it->first; break;
                }
                }
         }


    return 0;
}