#include <stdio.h>
#include <string.h>
int v[35];
char cuv[1000002];
int main()
{
    FILE *fin,*fout;
    fin=fopen("ordine.in","r");
    fout=fopen("ordine.out","w");
    int size,i,ci,ok;
    fscanf(fin,"%s",cuv);
    size=strlen(cuv);
    for(i=0; i<size; i++)
        v[cuv[i]-'a'+1]++;
    while(size)
    {
        ok=1;
        for(i=1; i<=26 && ok; i++)
        {
            if(v[i]==size/2+1)
            {
                ci=i;
                ok=0;
            }
        }
        for(i=1; i<=26 && ok; i++)
        {
            if(v[i] && ci!=i)
            {
                ci=i;
                ok=0;
            }
        }
        fprintf(fout,"%c",ci+'a'-1);
        v[ci]--;
        size--;
    }
    return 0;
}