#include <fstream>

using namespace std;

ifstream fin("ordine.in");
ofstream fout("ordine.out");

int a[26]={0};
int n,ok,k,s,i,j;
char b,z;

int main()
{

    fin>>b;
    while(!fin.eof())
    {
        ++a[b-'a'];
        ++n;
        fin>>b;
    }
    for(i=0;i<n;i++)
    {s=0;
    for(j=0;j<26&&s!=1;j++)
    {if(a[j]!=0&&j+'a'!=z)
            {
                a[j]--;ok=1;
                for(k=0;k<26;k++)
                if(a[k]>(n-i)/2)ok=0;
                if(ok)
                {fout<<(char)(j+'a');
                z=j+'a';s=1;
                }
                else
                a[j]++;
            }
    }

}
   return 0;
}