#include<stdio.h>
#include<fstream>
#include<string.h>
using namespace std;
int v[255],i,j,k,l,m,n;
char c,ant,ok;

int main(){


ifstream f("ordine.in");
freopen("ordine.out","w",stdout);


while(!f.eof()){
	f>>c;
	v[int(c)]++;
	if(c>='a'&&c<='z')
		n++;}

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