#include <fstream>
#include <cstring>
using namespace std;
ifstream cin("ordine.in");
ofstream cout("ordine.out");
char s[1000010],ch;
int N,fr[30],nr;
int main()
{
    cin.getline(s,1000001);
    N=strlen(s);
    for(int i=0;i<N;i++)
        fr[s[i]-'a']++;
    do{
            nr=0;
        for(int i=0;i<N;i++)
            if(fr[i])
            {
            if(fr[i]>1)
                nr++;
            ch=i+'a';
            cout<<ch;
            fr[i]--;
            }

    }while(nr);
    return 0;
}