#include <fstream>

using namespace std;

ifstream f("ordine.in");
ofstream g("ordine.out");

char c[1000005];
int fr[26];
int n;

int main()
{
    f.getline(c, sizeof(c));
    for (n=0; c[n]; n++)
        fr[c[n] - 'a']++;

    int p = 0;
    char u = 100;
    while (n--)
    {
        p = 0;
        while (!fr[p] || p == u)
            p++;

        g << char(p + 'a');

        fr[p]--;
        u = p;
    }

    return 0;
}