#include<cstdio>
#include<cstring>


#define MAX_SIZE 1000005

FILE *f=fopen("ordine.in","r");
FILE *g=fopen("ordine.out","w");

using namespace std;

char sir[MAX_SIZE];
int frec[29],size;
int ant;
void read ( void )
{
	fgets(sir,MAX_SIZE,f);
	size=strlen(sir)-1;
	for(int i=0;size;i++)
        ++frec[sir[i]-'a'];
	size++;
	
}
void solve ( void )
{
	ant=-1;
	for(int i=1;i<=size;i++)
    {
        int c=-1;
        for(int j=0;j<26;j++)
            if(ant!=j && frec[j]==(size-i+1)/2+1)
            {
                c=j;
                break;
            }
        if(c!=-1)
        {
            fprintf(g,"%c",char(c + 'a'));
            --frec[c];
            ant=c;
        }
        else
            for(int j=0;j<26;j++)
                if(ant!=j && frec[j])
                {
                    fprintf(g,"%c",char(j+'a'));
                    frec[j]--;
                    ant=j;
                    break;
                }
    }	
	
	
	
}
int main( void )
{
	read();
	solve();
	return 0;	
}