#include <iostream>
#include <fstream>

#define FIN "ordine.in"
#define FOUT "ordine.out"
#define N_MAX 1000000

using namespace std;

char a[N_MAX];

ifstream fin(FIN);
ofstream fout(FOUT);

int main()
{
	int t,n,flag,i;	
	
	fin >> a;
	n = strlen(a);

	for(i = 0; i < n; ++i)
		for(int j = i+1; j < n; ++j)
			if(a[j] < a[i])
				t = a[i], a[i] = a[j], a[j] = t;
	flag = 0;
	for(i = 0; i < n; ++i){
		if(a[i] == a[i+1]){
			flag = 1;
			continue;
		}
		if(flag){
			t = a[i],a[i] = a[i+1],a[i+1] = t;
			flag = 0;
		}
	}
	for(i = 0; i < n; ++i)
		fout << a[i];
	fout << endl;

	return 0;
}