#include <iostream>
#include <cstring>
#include <algorithm>
#include <fstream>

using namespace std;

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

char sir[256];
int aparitii[28];

void anagrama() {
	for (int i = 0; i <= 27; i++)
			for (int j = i + 1; j <= 27; j++)
				while (aparitii[j] && aparitii[i]) {
					cout << char(i + 'a');
					cout << char(j + 'a');
					aparitii[i]--;
					aparitii[j]--;
				}
}

int main() {

	cin >> sir;
	for (int i = 0; i <= strlen(sir); i++)
		aparitii[sir[i] - 'a']++;

	anagrama();

	return 0;
}