#include<stdio.h>
#include<string.h>
int v[30],i,j,k,l,m,n,ok,ant;
char text[1000100];

int main(){


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

fgets(text+1,1000010,f);
n=strlen(text+1);

for(i=1;i<=n;i++)
	v[(int)(text[i]-'a'+1)]++;

for(i=1;i<=n;i++)

	{ok=0;
		for(j=1;j<=30;j++)
			if(v[j]==((n-i+1)/2+1)){ok=j;break;}
		if(ok!=0){printf("%c",char(ok+'a'-1));v[j]--;ant=j;}
		else
			for(j=1;j<=30;j++)
				if((v[j]>0)&&(ant!=j)){printf("%c",char(j+'a'-1));v[j]--;ant=j;break;}
				
}
return 0;}