#include<fstream>
using namespace std;
ifstream fin("ordine.in");
ofstream fout("ordine.out");

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

int main(){
 
fin>>text;
 
 n=strlen(text);
 for(i=0;i<n;++i){
 ++a[text[i]-'a'+1];}
 
 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;
    
    fout<<x+'a'-1;
    --a[x];prev = x;--n;
    }
 
 return 0;
}