#include <stdio.h>
#include <string.h>
#define maxim 1000017
using namespace std;
int n,ma;
char T[maxim];
int i,j;
int A[41];

FILE *f,*g;

int urm(int de_unde)
{
	int i;
	for (i=de_unde+1;i<=40;i++)
		if (A[i])
			return i;
	return 0;
}

int main()
{
	f=fopen("ordine.in","r");
	g=fopen("ordine.out","w");
	fgets(T,sizeof(T),f);
	n=strlen(T);
	for (i=0;i<n;++i)
		A[T[i]-'a']++;
	i=urm(-1);
	j=urm(i);
	if (j==0)
		while (A[i])
		{
			fprintf(g,"%d",i+'a');
			A[i]--;
		}
	ma=j;
	while (ma!=0)
	{
		while (A[i] && A[j])
		{
			fprintf(g,"%c",i+'a');
			fprintf(g,"%c",j+'a');
			A[i]--;
			A[j]--;
		}
		if (!A[i])
		{
			i=urm(ma);
			ma=i;
			if (!i && urm(j))
			{
				while (A[j]!=0)
				{
					fprintf(g,"%c",j+'a');
					A[j]--;
				}
				return 0;
			}
		}
		if (!A[j])
		{
			j=urm(ma);
			ma=j;
			if (!j && i)
			{
				while (A[i]!=0)
				{
					fprintf(g,"%c",i+'a');
					A[i]--;
				}
				return 0;
			}
		}
	}
	fclose(f);
	fclose(g);
	return 0;
}