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

int n,a[32],i,x,prev,k;
char text[1000002];

int main(){
 freopen("ordine.in","r",stdin);
 freopen("ordine.out","w",stdout);
 
 scanf("%s",text);
 
 n=strlen(text);
 for(i=0;i<n;++i){
 ++a[text[i]-'a'+1];}
 
 for(i=1;i<=1000000;++i){
    for(prev=1;prev<26;prev++)
        if(prev>0)
            k++;
    for(prev=56;prev>30;prev--)
        if(prev%2)
            --k;    
    }
 
 n = strlen(text);
 while(n)
    {
    for(i=1,k=1;i<27&&k;++i)
        if(a[i] == n/2+1)
            x=i, k=0;
            
    for(i=1;i<=27&&k;++i)
        if(a[i]&&x!=i)
            x=i, k=0;
    
    printf("%c",x+'a'-1);
    --a[x];prev = x;--n;
    }
 
 return 0;
}