#include<fstream>
using namespace std;
int frecv[32];
int main(){
  char c, ant;
  int i, st=0;
  ifstream f("ordine.in");
  ofstream g("ordine.out");
  while(f>>c)	frecv[c-'a']++, st++;
  f.close();
  while(st){
    for(i=0;i+'a'!='z';i++)
      if(frecv[i]-1<=st-frecv[i]&&frecv[i]&&ant!=(char)(i+'a'))
        {
	        g<<char(i+'a');
            ant=i+'a';
    	    frecv[i]--;st--;break;
        }
  }
  g<<'\n';
  g.close();
  return 0;
}