#include<stdio.h>
#include<string.h>
char cit[1000010],a[30],*c,lt,cf;
long int i,j,fr[30],l,r,poz,k;
int main()
{
	FILE *f,*g;f=fopen("ordine.in","r");g=fopen("ordine.out","w");
	c=&cit[0];
	fscanf(f,"%s",c);
	l=strlen(c);
	for(i=0;i<26;i++)a[i]=(char)i+'a';
	for(i=0;i<l;i++)
	 fr[(long int)(c[i]-'a')]++;
	for(i=0;i<26;i++)
	 if(fr[i]) { fr[j]=fr[i];a[i]=a[j];j++;}
	a[j]=0;j--;
	if(fr[j]>l-fr[j])
	{ fprintf(g,"%c",a[j]);fr[j]--;
	  poz=0;
	  for(i=1;i<=fr[j];i++)
	   { fprintf(g,"%c%c",a[poz],a[j]);fr[poz]--;
	     if(!fr[poz])poz++;
	   }
	  fprintf(g,"\n");
	  fcloseall();
	  return 0;
	}
	if(fr[j]==l-fr[j])
	{ poz=0;
	  for(i=1;i<=fr[j];i++)
	   { fprintf(g,"%c%c",a[poz],a[j]);fr[poz]--;
	     if(!fr[poz])poz++;
	   }
	  fprintf(g,"\n");
	  fcloseall();
	  return 0;
	}
        for(;;);
	fprintf(g,"\n");
	fcloseall();
	return 0;
}