#include <fstream>
#include <cstring>
using namespace std;
ifstream fin("ordine.in");
ofstream fout("ordine.out");
int n,fr[30];
char s[1000005];

void Solve(int last)
{
    int i;
    bool ok = false;
    for(i = 0; i < 26; i++)
    {
        if(i!=last && fr[i] > n/2)
        {
            fr[i]--;
            fout<<(char)(i+'a');
            ok = true;
            last = i;
            break;
        }
    }
    if(!ok)
    {
        for(i = 0 ; i < 26; i++)
        {
            if(i!=last && fr[i]>0)
            {
                fout<<(char)(i+'a');
                fr[i]--;
                last = i;
                break;
            }
        }
    }
    n--;
    if(n>0) Solve(last);
}
int main()
{
    int i,j;
    fin>>(s+1);
    n = strlen(s+1);
    for(i = 1; i <= n; i++)
        fr[s[i]-'a']++;
    Solve(100);
    fout.close();
    return 0;
}