#include<stdio.h>
#include<algorithm>
#include<string.h>
using namespace std;

char S[2000100];
int A[301];
int nr;
int P;

int swap(int a,int b)
{
	int c = S[a];
	S[a] = S[b];
	S[b] = c;
}

int main()
{
	FILE *f = fopen("ordine.in","r");
	FILE *g = fopen("ordine.out","w");
	
	fscanf(f,"%s",&S);
	nr = strlen(S);
	sort(S,S+nr);
	for(int i=0;i<nr;i++)
		if(!A[S[i]])
			A[S[i]] = i;
	for(int i=1;i<nr;i++)
		if(S[i] == S[i-1])
		{
			int j;
			for(j=S[i]+1;!A[j] && j<='z';j++);
			if(j < 'z' + 1)
			{
				P = A[j];
				if(S[A[j]+1] == S[A[j]])
					A[j] ++;
				else
					A[j] = 0;
				swap(i,P);
			}
		}
	int k = 0;
	int j = 0;
	for(int i=nr-1;i;i--)
		if(S[i] == S[nr-1])
			k ++;
	for(int i=nr-1;S[i] == S[nr-1];i--)
		j ++;
	for(int i=nr-2;i;i--)
		if(S[i] == S[nr-1])
			S[i] = 'z' + 1;
	sort(S+nr-1-j-2*(k-j),S+nr-1);
	for(int i=0;i<=nr-(2*j+2*(k-j));i++)
		fprintf(g,"%c",S[i]);
	for(int i=nr-2*(j+(k-j))+1;i<=nr-2*(j+(k-j))+k-1;i++)
		fprintf(g,"%c%c",S[nr-1],S[i]);
	fprintf(g,"%c",S[nr-1]);
//	fprintf(g,"\n%s %d %d",S,j,k);
	
	fclose(g);
	fclose(f);
	return 0;
}