#include <iostream>
using namespace std;
#define UNDEFINED 32432523
int apar[50], k, v;
string s;
char rez[10000];
int find_abundent() {
    int maxim = 0, value;
    for(int i = 0; i < 27; i++) {
        if(maxim <= apar[i]) {
            maxim = apar[i];
            value = i;
        }
    }
    return value;
}
int main()
{
    cin >> s;
    for(int i = 0; i < s.size(); i++)
        apar[s[i] - 'a']++;
    for(int i = 0; i < s.size(); i++) {
        if(rez[i] == 0) {
            char current_char = find_abundent();
            int ind = i;
            while(apar[current_char] != 0) {
                if(ind >= s.size())
                {
                    cout << char(current_char + 97);
                    return 0;
                }
                rez[ind] = current_char + 97;
                ind += 2;
                apar[current_char]--;
            }
        }
    }
    for(int i = 0; i < s.size(); i++)
        cout << rez[i];
}