#include <fstream>
#include <algorithm>
#include <cstring>
using namespace std;
ifstream cin("ordine.in");
ofstream cout("ordine.out");
char s[1000010],ch;
int N,fr[30],nr,ant;
int main()
{
    cin.getline(s,1000001);
    N=strlen(s);
    for(int i=0;i<N;i++)
        fr[s[i]-'a']++;
    for(int l=0;l<N;l++)
    {
        for(int i=0;i<26;i++)
            if(fr[i]&&(l==0||ant!=i))
            {
            fr[i]--;
            ant=i;
            cout<<(char)(i+'a');
            break;
            }
    }
    cout<<'\n';
    return 0;
}