# include <stdio.h>
# include <string.h>

const long int MAXN=1000000;
long int count[100],n;

void citire()
{
char s[MAXN+10];
FILE *f=fopen("ordine.in","r");
fgets(s,MAXN,f);
fcloseall();
long int i=strlen(s)-2;
n=i+1;
while (i>=0)
	{
	count[(int)s[i]-(int)'a'+1]++;
	i--;
	}
}

void scrie()
{
FILE *g=fopen("ordine.out","w");
long int old=0,j,i,ok;
while (n)
	{
	ok=0;
	for (j=1;j<=(int)'z'-(int)'a'+1;j++) if (count[j]==n/2+1)
		{
		fprintf(g,"%c",(char)((int)'a'+j-1));
		count[j]--;
		n--;
		old=j;
		ok=1;
		break;
		}
	if (ok==0)
		{
		//if (!old)
			{
			i=1;
			while (count[i]==0||i==old) i++;
			}
		old=i;
		count[i]--;
		fprintf(g,"%c",(char)((int)'a'+i-1));
		n--;
		}
	}
fprintf(g,"\n");
fcloseall();
}

int main()
{
citire();
scrie();
return 0;
}